trace back the path of your engine wiring harness where it goes throught the firewall. Unscrew it, get a mini-hack saw and make two cuts and bend back the sheet metal. I routed two 4-gauge wires along with some other wires through. The wires will end up underneath the passenger kick panel, you can then run it under the lower rail trim (pop up the trim, peel back carpet, run it along with the car wiring harness wires already there) , and into the trunk. This method assured that i would not pinch any wires or underneath the car where it could be damaged.
Ive done this to two cars with no problems, ask if you have any questions, i have more picks of my stereo install. This is how the pros do it.
1999-2007: ___1988 Bonneville LE LN3 (Vin C) @ 273,000 miles
and still going strong (<-Faded sun fried grayish ie needs paint)
2007-Present: ___1993 Bonneville SE L27 @ 210,000
Speeding around So Cal (<-Beautiful Forest Green)
Mods: Sylvania Silver Star headlamps. Remote start system with pageing device. K&N air filter. Tranny Cooler. High-flow cat with Magnaflow muffler. Full Class II hitch setup.