Just remember that the factory headunit most likely will have to stay hooked up with the car(relocate it to the trunk or somewhere you can find space) or otherwise the car might start to do weird things.
Unhook the factory wiring harness to the stock radio and drive it around for a few days and see if you all the sudden lose the ability to remote unlock/lock the doors, no interior lights until you put the key into the ignition, dash lights all starts to flicker and/or factory alarm goes off for no reason. Oh and the door chime come out of the factory headunit and into the front left speaker so you'll lose door chime. They might make a CORRECT interface module with databus hookup(they didn't before) but their built-in door chime sounds like crap.
I had to do the same with my Deville 5 years ago and let me tell ya... it was a PITA... it ****** a lot more because I had to figure it out myself.