battery

1. Parts needed:

Grab a battery cable out of an e30 that runs from the trunk to the front of the car. Also grab the plastic thing that holds the wire to the firewall (you'll need this). 2 3/4 inch (inside diameter but for a 1 inch hole) rubber grommets at home depot, silicon sealant, negative ground cable without terminals, and about 36 inches of battery cable, a battery box (unless you're doing the brace). Drill saw, 1 inch diameter, and of couse, a drill. You might also want an extension bar of some sort for the drill. It'll help big time when drilling the front hole needed.

2. Hole drilling:

You'll need to drill 2 holes, both one inch in diameter. One in the firewall right near where the wiring harness passes through. On that hole you'll have to cut through insulation on the engine side, the firewall, then through what seems to be a cardboard plyboard. Do this drilling from the engine side then into the car, this will prevent you hitting the brake lines if you go the other way. The other hole needs to be cut through the trunk through the area of the back seat. To get that hole location correct, while in the trunk drill a hole right where the rear firwall meets the trunkbed on the driver's side of the car.

3. Put it together

Take the rubber grommets and put them in the holes you just drilled (see above pics). Feed the cable under into the back seat compartment. There's a hole in the back seat down by the floorboards on the drivers side that you'll run the cable through. Start ripping up the carpet on the drivers side. The cable is going to run along the drivers side rocker panel inside the car.
Pass the wire through the next hole Once the wire is through, it's best run through a gap between the fender wall and the wiring harness. Then run it stright along under the brake fluid resivoir.

That plastic thing is now needed. Drill 2 small holes into the wheel well to affix the plastic cable holder right next to where the voltage regulator sits. Get the wire in there, and then place your existing positive terminal on the bullet end. Take out your existing negative terminal and the battery tray. Run a negative wire from the exisitng ground area on the block and then to the ground spot right where the battery tray used to sit.

4. Back to the Rear

To set up the battery, first line up the battery box strap secure clamps in the middle of the engine, easy part. Then get the battery in there. Run that other battery cable as a ground to the bolt where the trunk lid attaches to the body of the car, there are two on the driver's side, I used the one closest to the front of the car. Get that silicon sealant out now and seal up the battery cable where you ran it through grommets. Now put the terminals on the battery and start her up.