For those of you with batteries in your trunk

[projectRDM]

If you have a sealed type battery, like an Optima, buying a relocation kit is a waste of money. You can buy a plate from any auto parts store, and enough wire from a welding supplier or a stereo installation facility and save money, plus most battery boxes are very large and take up more room. If you do not have a sealed battery, it's highly recommended not to mount it inside the vehicle unless you're using a vented type box that actually seals airtight and lets gases escape under the car.

On either car, it's best to mount it on the passenger side, it offsets the weight of the driver and allows for less wire run overall. As already implied, S14s have a perfect place with bolts already existing to mount to, the bolts are there to secure the floorpan to the framerail before final assembly, you can remove them without worry of any harm. S13s can be put more towards the tail of the car, or against the shock tower, either place is fine and unless you're using 2 inch bolts/screws you're not going to hit the tank. The fuel pump access panel should be avoided though.

Running the wire inside or out is up to the owner, I prefer going outside because there's less work to get the wire through the car, you avoid a bulge in the carpet, and the brakeline makes a perfect place to tie to, tucked behind the framerail and hidden from view or damage. Plus running the power wire near the ECU may introduce feedback/noise into the engine's operating system, depending on age/condition of the car. Both cars have access plugs in the back to run the wire through.
A good coat of rubberized undercoating after the wire is installed will provide even better protection and keep it dry and corrosion free.