Disclaimer: If you attempt this mod to your car and if by some freak accident something goes wrong and your car burns to the ground, you get seriously injured and your heart stops beating, or you need any sort of medical attention from doing anything that relates to what I am telling you to do. I hope you don't come looking for me to place any sort of blame or wrong doing because I am not forcing you to install anything on your car but just trying to give you some guidance. As with anything that has to do with electricity try to not touch any exposed wire that has current running through it or anything else that might seem kind of like you shouldn't do when working with electricity (Remove all jewelry and try your best not to kill yourself). I highly doubt that anyone could mess this install up and accidently kill themselves but I figured I would post a disclaimer just incase someone found a way (those people would be the some ones that most of the warning labels are wrote for so they don't try sueing for their own stupidity).
Some advice and things to consider......
As a deterant towards relocating your battery to the trunk I would suggest you make all attempts to keep the battery in the engine bay unless you can avoid doing this modification to your car. I personally needed the space in the engine bay and had basically run out of room and needed to start moving things around and taking stuff out so if you gotta do this then I will attempt to make this as painless as possible. Depending on how many tools you have at your disposal this shouldn't be too difficult even for those with a limited amount of tools. I will try to cover which tools are required in each step but I am writing this How-to from memory so if I miss any tools or screw up the size chances are it will be easy enough to figure out which ones to use. This How-to is wrote for a 1997 ACR Neon so some of the bolts in various parts of the car might not be the same for your car (unless you aren't even doing this on a Neon then you are on your own on certain things). This How-to is also wrote using specific battery relocating parts so if you choose not to use the ones I spec out then you will have to make adjustments where needed to make it all work. Make sure you have a battery box (Jegs PN#555-10240), the basic battery relocation kit (Jegs PN#555-10303), 4 gauge power & ground wire (20 feet of power wire & 20 feet of ground wire), crimp terminals for 2, 4, 8, & 10 gauge wire, lots of zip ties, rubber grommets, silicone gasket maker, and atleast 20+ feet of convaluted tubing to cover the wires (and a partridge in a pear tree). Make sure that you read this how-to several times to see what you are getting yourself into and make sure you allow yourself plenty of time to install everything because it is really time consuming and it ain't cheap by any stretch of the imagination because the 4 gauge wire I bought cost me damn near $2.50 a foot and you need 40 feet of the stuff. Another tip is to make sure you have everything you need on hand and then make sure that you have doubles of everything that you think that you might accidently break so you don't have to go hunting for the parts during the install, which I am sure comes as no surprise to anyone who has been around Neons for any length of time and has done a "few" mods in the along the way and broke a few important components during an install (yeah it really sucks).
Step: 1
Remove the battery and battery tray....relatively easy stuff but this is only the beginning. I didn't get any pictures of how to remove the battery and tray so if you need these pictures and detailed explanation on how to remove the battery and tray then you really should reconsider doing this because you can always turn back but as you go it gets difficult to do so. If you are bull headed like me and want to go on anyway and learn as you go then I suggest getting a FSM (Factory Service Manual) and reading up on how to remove the battery and tray. Tools needed for this step are adjustable wrenches, rachet, extensions, and possibly a 15mm socket (not sure on the size of the two bolts that are holding the tray to the frame but I think they are 15mm).
Step: 2
Next you need to remove the power and ground wires that were attached to the terminals of the battery. The power wire will be connected in two spots - Power Distribution Center and starter motor. The ground wire will also be attached in two locations - The frame above the left radiator fan and a bolt that goes to the transmission above the starter motor. Make sure that you only remove the two wires on the starter motor that are needed to be removed becuase there are actually two other wires (one to each terminal) that need to stay where they are. Tools needed are sockets (8mm and either 13mm or 15mm for the bolts on the starter motor and transmission) and rachets.
Step 3:
This step is the most fun because you get to rip out the entire passengers side and rear interior of the car. You need to remove the plastic trim on the lower door frame, which should need a flat blade screw driver to gently pry up on it to get it off. Next you remove the rubber gasket that seals the door from the outside. After that you take off the trim piece in the passengers foot well, which should only need to be pryed loose with your fingers. Then you remove the rear bench seat cushing, which should just come right out with little or no hassle. Then you need a T-55 Star bit, extension, and a breaker bar to remove the rear passengers seat buckles. Next you push up from the bottom on the rear seat back rest until it comes to a stop and then you pull it straight out from the frame. After that you need to remove the passengers seat, which should use an 18mm socket and either a breaker bar or a large ratchet and possibly an extension depending on if you use a deep socket. Once you have done that you need to remove the passengers seat belt bolt from the frame which should require a T-50 star bit and a breaker bar. Then you need to remove the passengers side coat hook above the rear passengers side window, which should require a T-20 or T-25 star bit and a small rachet or screw driver handle that can accept assorted sockets and star bits with the proper adaptors. Next you need to remove the entire passengers side rear panel (big piece around right rear window) which should only require you to pull it up with your fingers. Once that thing is free you need to push both the front and rear seat belts through the panel to remove it from the car. Finally you need to take the carpet up on the passengers side of the car and pile it up on the drivers side of the car. A tip to keep the carpet from falling back into place is to take the hole that the passengers seatbelt went through on the carpet and push it over the e-brake handle.
Step 4:
Next you need to decide if you want to keep the carpet in your trunk or not. Because one way or another you need to remove it to drill the holes for the battery box mounting bolts, which are actually long threaded rods. So you need to figure out where you want the box located in the trunk and then proceed to drill the holes in your trunk floor for the rods/bolts. Then all you need to do is install the hardware and put the box in its final resting place. About all the tools needed for this step is a drill, bit (sizes may vary), and a pair of adjustable wrenches.
Step 5:
Here are some tips and concerns for drilling through the firewall before you go crazy and start making the firewall look like swiss cheese. First off you need to determine where you can and cannot drill the hole in the firewall, which isn't really very easy. As a tip for keeping the drill bit from walking around use a hammer and a set punch first to get the placement of the hole worked out. From there use a small drill bit and step up in sizes from there until you reach the size you want so as not to wear out just one bit (you still have 2 more holes to drill).
The problems/concerns with drilling through the firewall:
If your coolant overflow bottle is in the way you might drill through it.
If you aren't careful you could drill through your fuel line.
Also if you aren't careful you could drill through your brakelines (if I remember correctly)
You could accidently drill into the cavety that holds the lower control arm cage nut, which is not bad but it isn't helping you with running the wires since it doesn't get you into the engine bay.
From what I can remember Rich drilled the inital hole for the first wire about 3-1/2 to 4 inches over from the wheel well hump as seen better inthe picture below. The other holes that I drilled in the firewall were within an inch of the first hole. In total you should have about 3 holes drilled in the firewall for the different wires. Tools needed for this step are a drill, several huge cold steel drill bits ($$$$), hammer, and set punch.
Step 6:
The battery box from Jegs should already come with two holes and grommets but you will need to drill 2 more holes for the extra wires that will be needed for this application. For ease of drilling you should try to find the drill bits that are the exact same size as the holes already drilled in the box. From there you need to source 2 more grommets (probably more incase you destroy a few in this step (I did). So the holes you need to drill in the box should be atleast 1 inch away from the ones already in the box. Tools required for this step are a drill, bits, hammer, and set punch.
Step 7:
After drilling the holes in the battery box the next step is running the wires from the PDC and Starter motor to the trunk. The 2 gauge power wire goes from the positive pole of the battery to the PDC. The 4 gauge power wire goes from the positive connector on the starter motor to the trunk. The only tools needed for this step would be your own two hands. Also as a reminder you need to make sure that you run the wires through the battery box before you do anything with crimping or soldering because you will regret it.
Step 8:
Next you need to run the 2 gauge ground wire that goes to the grounding point on the transmission that the previous ground wire that you removed from a different step went. The last wire needing to be run just goes from the right rear tail light ground point to the negative pole of the battery. Again the only tools needed for this step is your two hands. Remember to run these wires through everything before you do anything with the crimper or solder. Sorry for the lack of pictures of the ground wire. The reason for this is I was busy trying to get everything done before it got to dark to work on the car and I really didn't have the time to stop and mess with the camera to get the shots needed. The wires basically follow the same path from the box to the engine bay.
Step 9:
The next step is to install crimp connectors or solder connectors on both ends of power wires and ground wires after the length is finalized and excess is cut off. I would personally stay clear of the soldered connectors since soldering has been known to weaken the wires and any sort of bending could snap the wires. So I suggest the crimp connectors and a really big ass pair of vice grips. The guy I talked to on how to install them told me to use a hammer and a big set punch or chisel. I used the chisel method on the first one and damn near destroyed the connector. After that I used the big pair of vice grips and it worked out the best since I could control the force applied to the connector.
Step 10:
If you want to keep insects, water, etc.. from getting through the holes you just drilled through the firewall then it is recommend that you put silicone gasket maker around the wires. This will stabilize the wires to keep them from rubbing on the firewall and eventually shorting out, which might be bad if it does occur.
Step 11:
The next step is to secure everything with zip ties. Basically before you do any of this you need to make certain that your wires are not rubbing on anything, have the potential to rub on anything, or are in a place that they might come in contact with something hot and possibly melt. Make sure to zip tie the wires down every 12" or so to something but make sure there is some slack in the wires so they don't get destroyed (especially the two that are mounted to the engine). Finally install convoluted tubing over wires to protect the wires and make an attempt to dress up the engine bay.
Step 12:
The final step before you put everything back together I recommend hooking up the battery and seeing if the car will start or burn to the ground. If you did everything right you shouldn't have any problems.