Radio?

[f5fstop]

Ok, base radios come with a built in amplifier to supply about 40 watts to the speakers. If the six-disc is designed for a Monsoon system, it only outputs minimal wattage (about 3-10) to supply a signal to the Monsoon amplifier. The amplifier supplies the higher wattage to run the speakers. So be careful what you purchase. (Not sure if a six disc is offered without Monsoon or not.)
You can install yourself, very easy, about 20 minutes if it is the first time. Just pull the trim panel, remove the radio screws, unplug connectors and antenna, then reverse to install new radio. The wiring for all radios are the same at the radio head.
The only thing the dealer will have to do is program the radio to the BCM and the VIN from the vehicle into the radio using a Tech 2. The vehicles VIN is required due to theft protection. The radio looks at the VIN supplied by the BCM, if it is not correct, it will show "LOCK."
The dealer will be able to look up the part number for the XM receiver; it won't be cheap.

h3'er is correct, an aftermarket with Xm might be less expensive.