Shouldn't be all that difficult. Easiest way is to run the RCA cables down the passenger side of the car. You should be able to lift the plastic covering the bottom part of the door jamb where the carpet touches it and the pasenger footwell panels (these are all usually held on by metal clips). The same goes for the rear of the car. Make sure the power and RCA cables are exlusive of each other and well shielded or you'll have EMF bleed over.
the best way to avoid interference between power lines and interconnects and speaker wires is run the power along one side of the floor (left) and interconnects and speaker wires along the other side (right). yes pop out the footwell panels (like they had to do in the french connection to find all of the drugs) and you will find a great place to run cable to the back.
for a 1st timer its very easy with the right tools and patience.
Well, the bmw dash is very tight behind the radio, you just have to feed them behind the radio and down behind the dash toward the kick panel, then run them under the door sills and carpet back to the back seat, then take the back seat out to get them through the trunk...