I'm in the middle of wiring an amp going inbetween my two subs under the rear bench seat. Where's the best spot to ground the amp back there? I want somewhere that will have a solid grounding with no visible wires whatsoever.

 

I made a ground under the child seat restraint loop bolt with a big O-ring terminal. Also under the door floor panels are plenty of holes to run a ground right to the frame. Since your mounting under the seat just loosen up a seat bolt and slip a terminal between there. Rub the spot raw with sand paper first to assure a good ground.