alright dude.
take the grounds off the amps, run them under the false floor like you did for your power wire, and up the inside of the wheel well area toward where the C pillar meets the rear deck. there is something called a strut tower there. basically its a very strong and reinforced frame type area. its a structural support for the rear suspension. there are 3 bolts that hold the suspension to the frame of the car. get some nice fat ring terminals, undo one of the bolts, route your wire up through the wheel well area to the side rear deck that is closest to your amps, and sand\scrape the paint off and ground it there.
i had a newer SL2 and it worked very well. all you gotta do is remove the rear deck wich takes only a few minutes, and you will see the three bolts on each side of the rear deck. and it should be very easy to route the wires up through the corners of the rear deck from the trunk.
its just that the rear deck that you grounded to is very thin, and there is not much surface area with that bare wire you are trying to attach, eventually it WILL fall off. that would be bad. so crimp some fat *** connectors on that 4 and 0 guage wire and solidly mount them to those strut tower bolts. EVERY car has them and they have always worked great for me.