2004 GMC Envoy GSP Unit

I have a 2004 GMC Envoy SLT with Bose System and I bought a GPS unit from ebay to replace the 6 disk changer but once installed all options but the volume work - it is very very very low with a buzz sound.

I have heard that when you put a new unit in you have to go to the dealership to have the unit unlocked but I can go through all the units options with no problem. Also, the prompt from the GSP comes through the speakers clear and loud. Does anyone know how fix this problem or have any idea what the issue is?

Thanks