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It is my understanding that all H3 radios are XM capatible; therefore, any H3 radio can be fitted with the XM receiver.
If you wanted a factory type conversion, you would need to purchase the XM receiver module from the dealer, and you may or may not require a new antenna.
The XM receiver installs in the car and then connects to the radio and vehicle power/ground. The H3 comes with an onstar antenna, which is also used for the XM radio. However, some have said that it does not come with both antenna leads unless from the factory with XM. Therefore, you would need to purchase a new antenna with both leads.
After the XM is installed and/or the radio is chaged out in an H3, the dealer has to program the system, so that the radio does not lock and the XM will operate. All H3 radios need the VIN of the vehicle they are installed in, programmed into their system. When you first turn on a radio, it looks at the VIN being supplied by the BCM, if the VIN is different, it locks the radio.
I believe there are aftermarket XM receivers that can be wired into the factory radio, and they operate exactly the same as the factory XM receiver (displaying all information on the radio head).