There is 2 ways Ive done it:
1st - At the bottom of the spare tire wheel (removed of course) I drill a hole, usually 1.5" and use that plastic stuff (also comes in chrome or gold... for those bling-blingers) that runs along the door edge, protecting your doors from damaging another. I put super glue inside the lining and run it all the way around the edge of the hole. Why 1.5"? Ive had nitrous in all my other cars... good for upgrades
2nd - run the 1/0 gauge back into a distribution block under the car. Instead of 1x 1.5" hole.. i do 2x 1" holes (or more...) for 4 gauge to go to the amps. One BIG advantage to this is: replace fuses quickly. Quick disadvantage: Where will it be mounted? On my Jetta, I removed the spare tire well and replaced it with a flat sheet of metal.
It is not wise at all to run any wire larger than 4 gauge inside the cab. If anything were to happen to the power wire... it will burst into flames. If your running 4 gauge or bigger, that means your main fuse (near battery) will be over 100 amps. That kind of power will FRY...
Im going to be running a Phoenix Gold Octane 8:4.0 and a Cadence A7HC. The Phoenix puts out 125x4, and the Cadence dishes out 1400wx1. Both amps run 80amp fuses.