I have several factory service manuals for several different year series of caddillac's and every one says that any aftermarket items that are to be added to the vehicle is to be wired directly into the battery- not into any other place in the vehicle. I think it has to do with the sensors and the computer system in the car.
I got my wiring harness and dash kit from wal-mart. As long as you have the Base radio (i.e. non - Bose) there is no need to relocate the radio to the trunk. I installed my pioneer HU and wal-mart harness and dash kit in 30 mins.