Wendell,
From the factory, the NAV system has its own GPS antenna, mounted in the dash. From the factory, the OnStar GPS and the NAV GPS are separate units.
Remember, I was one of the first to do this setup. If I had to do it all over again, I'd probably just add a magazine type changer and mount it in the console or under the seat, saving the OnStar. It would have been a lot easier!
About the Theftlock... When a new radio is installed, the first time it is powered up, it "learns" the VIN of the vehicle. After that, each time it is powered up, it compares VIN of the current vehicle with the "learned" VIN number. If they don't match, the unit displays "Theftlock Activated" and will not power up. Therefore, if you buy a new unit, just plug it in and you are good to go. However, if you buy a used unit, you will have to get a dealer connect his Tech2 tool to command the radio to "relearn" the VIN from the current vehicle.
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