2004 GMC Envoy GSP Unit

I have a 2004 GMC Envoy SLT with 6 disk Bose System which I recently replaced with a 2004 Nav Unit I bought off ebay. Once installed the volume throughout the truck including the rear DVD system stopped working. The steering wheel controls work (except volume), Nav works and the prompt comes throught the speakers load and clear. When the station is changed volume comes through for a spilt second and when the engine isnt running you can hear the radio really really low with a buzzing noise.

I have heard that when you put a different unit into your vehicle it has to be reprogrammed by the dealer - is this true? would I be able to scan through the options etc.if this system was locked?

I have also heard that it could possible be muted from the steering wheel or an OnStar problem.

Any help would be much appreciated.