my cd player in my 03 Envoy is skipping more and more lately. I am looking at updating the head unit and was wondering which parts i need. I know the wiring harness and mounting kit are required, but are there any other modules i need to retain certain features (stereo running while car is off, door chimes, etc.) Plus, it is a 6 speaker system, will i need to add an amp to retain this or is there a way to run 6 off of a 50x4 head unit? Thanks for your help folks.