Cold Fusion, my amp wire goes through the firewall behind the battery. From the inside, it goes through above the carpet, behind the brake pedal. I assume this hole was there all along, but I didn't look before the guys at Tweeter installed my amp and sub. The firewall is really the most difficult part, after that, it's just running the wires under the carpet by the doors, under the back seat and into the trunk.
I have a SEL V6 with the premium audio, not audiophile, but I'm sure the wiring is similar. I had them add a speaker level to line level converter prior to the sub, as they felt it provided better sound quality. If you take it to a reputable shop, they can do it with no problem. If you do it yourself, just be patient and start in the engine compartment and work your way back. Most of the amp wiring fuses won't fit through the firewall. Also, check Crutchfield as they have lots of wiring diagrams to go by. Good luck- it's worth the effort to add a sub and amp.
I am planning on getting my subs hooked up really soon and then the neighbors will not like me again!
