The radio wire harness diagram

For the 03 P5, they have the little spare tire sub come from the back of the trunk, I just wonder that if i wanted to change it to In dash DVD unit, anybody know how to identify each wires that connect to the harness, so i could just tap it with other wire and just plug right in to my DVD unit and the Amp that i bought. Anybody has that diagram explain the wire on the harness for both front and back, the front is connected to the radio, and the rear is connected to the spare tire sub.....THanks guys(drinks)