#1.
I really don't know. Usually there are places to pass through near where the pedals go into the firewall... if you can't find a hole you can either make one by using a hammer and hammering where you want to drill the hole so you don't drill anything important, or you can usually pass the wire through the fender, from the door jam to the area that the hood latches on to your car.
#2.
You need to run speaker wire from your amp to your speakers. The head unit speaker wire out will not be used, only the rca's. A trick to use is to cut the speaker wire behind the head unit, not from the doors, so you don't have to rewire everything. You can hook up the amp's speaker wires to where you cut the wires. This will eliminate you needing to take your door panels off to rewire your front speakers.
#3.
Usually you use the + from one channel and the - from another, but this can be different with each amp. Check the manual and it will tell you. But remember, bridging is not always better. Check to make sure that you are wiring your speakers to the correct resistance before you go bridging your amp.
#4.
You can ground the amp pretty much anywhere you want. Everytime I've grounded an amp in the trunk, I've either used a pre-existing bolt from something under the carpet in the trunk area, or drilled a new hole and put a bolt in it. Just remember to get a clean ground on your system, and sand off any paint that could get in the way. The ground is a crutial part of your system, and you don't want it to be barely functional.
If you need any help, don't be afraid to ask... and most of your questions can be answered here:
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