Just bought a new 2500 HD WT and came with AM/FM radio only. Can I just find a stock AM/FM/CD/XM unit (provided I add a XM antenna) and swap it out with no problems? Anybody have a contact number to call for GM that can give me that answer. The dealership is taking the easy way out and saying no. I just can't take them at their word because the units look the same and should be an easy swap.