Rick Paulin, writing in the BMW Digest:

In response to your question about relocating the battery to the trunk. I too moved the battery to the trunk in my 74 2002 about 13 years ago. I forget what gauge the cable is, but I also got it from a welding shop. I ran it through an existing hole in the firewall (just added a grommet), along the drivers door under the carpet, into the trunk between the rear wheel well and the fender. I drilled a hole in the floor and fastened it under the car for grounding. For mounting, I used angle iron, the battery box sits between, and I have a strap running from the back of the angle iron to the front over the battery. The carpeting in the trunk pretty much hides the mounting bracket. With this application, your not limited to the battery for a 2002, get one next time with more cold cranking power.

Greg Mierz, also in the digest:

Jonathan writes about how to mount his battery in his trunk. First off, mine been in the trunk for 10years. Try to find bulk OO/ or OOO welding cable for the positive cable, its much more bendable and it of bigger size to compensate for the longer run of wire. Run the positive cable inside the car with grommets whenever the wire goes thru metal (a short here could fry the car). On my car I ran it along the driver side sill under the carpet over the pedal assembly and out thru the firewall just above the starter. Ground the battery in the trunk, I used the unused shoulder strap fitting on the under side of the package tray but any real clean solid spot will work. BE SURE the ground straps between motor and frame are in good shape under the hood. It would be wise to check all the ground under the hood at this time. A good marine case will hold the battery but have a good way to secure the case in the trunk, Angle iron , straps or whatever but make sure it would move around. It's a good idea to have it so that you can slide it out a little for maintance so leave some extra length to the cables. A welding shop will also have the right fitting for the cable ends, then just use wing nut battery posts to hook up the cables. Good luck.

And finally, Ian Dickerson:

I used a combination rear shock tower strut brace/battery pan from Import Engineering to mount the battery in the trunk of my '76 '02. The brace looks like an inverted triangle, with the battery frame on the bottom. This keeps the battery ahead of a line drawn between the two shock towers, and leaves most of your trunk left for storage. This is a neat solution to the battery dilemma. I use an Interstate Group 42 battery, and haven't put it in a box or anything. I used #2 gauge wire (got it at a welding shop), and ran the wire through grommets in the backseat bulkhead, and then inside the cabin, under the driver's seat and through a pre-existing hole in the firewall to the starter.