my own stereo insallers did a half-@$$ job of running my wires for my amp, they clearly didn't bother to remove the back seat. i went back and removed it and pretty much re-did everything they did, -only better.
Definitely remove the back seat, run your wires correctly. My 1997 bonneville had a backing panel made of pressboard behind the seat back. I got a plastic one off a 1994 in a junkyard that is vastly more sturdy and replaced my fiberboard one with the plastic one. Yay, upgrade that nobody will ever notice except me!