If you take out your HU, it has a label on the topside with a color diagram explaining the wires... and I would not cut any wire in a new car, simply splicing into it is fine.
As far as running wires thru doors, I like to use the heavy duty zip ties from Home Depot.
.:R32 - DÜ WERK - masDUBation
Please drive hybrids, I need the gas.