I talked to some of the mechanics working at my dealer and they told me most people go through the firewall by using a clothes hanger to punch trough the rubber gasket that lets the wiring go to the engine compartment. Here's how I did it...
I straighten a clothes hanger and from the engine compartment I used it to punch a hole in the rubber gasket:
Be careful when you punch the gasket, there are 2 layers of rubber, go straight trough the first one and then when it gets to the second layer of rubber get it away from the cable harness so you don't risk scraping the car wires!!!
Here's a photo of what it looks like from inside the car, driver side under the dash:
Then I used some duck tape to attach the power wire to the clothes hagner (about 3 to 4 inches of cable so its sturdy enough to go through the rubber gasket):
Then I pulled on the clothes hanger from the inside, carefully because you don't wanna pull the gasket away from the hole in the firewall:
I used a Schosche AMP kit rated for 160 watts from Walmart and its got a 12 gauge power wire.. as you can see it fairly big with the tape holding it to the clothes hanger so you gotta be _very_ careful when you pull it trough the rubber gasket! :
Netx, I'll be posting some photos and a how to to get all the wires to the back of the car and connecting the head unit to the sub.