I don't know about a '90 one... but on the '86 (older model :/), the battery was in the back left corner (looking towards the engine), and on that side with the door, there was a small notch where it was possible to run wires through,and enter them into the doorway and down into the carpets without affecting how the door opened. Covered in black tape (gorilla tape ftw), the wires weren't noticeable, and we avoided drilling.
Problem is for anything big, you'd have to make multiple runs of smaller gauge. I don't think 1/0 would fit, and we didn't even try 4, we just used the six gauge, since it was only an amp for his four speakers running around 300 rms.