BMW 318 Forum : “how to get amp wires into the car”

I would suggest trying to find the existing factory amp and draw power from that.  

most of the time you drill,sometimes you get lucky and have room around the break pedal,vents or so on.chances are you will have to drill.

Thats easy since your battery is already in the trunk. Just run a short amp wire from the battery to your amp and ground it to your body some were and your done. Just need to run your remote wire and rc jacks to the amp.