this site shows you how to jack in to the gps receiver inside any gm onstar system. it’s as simple as soldering a up a serial cable. you can then connect to it and either run some gps diagnostic software, or switch the device to nmea mode so that you can use it with your gps mapping software in your car pc.
if you’ve got an onstar system but aren’t paying for the service this might be just the hack for you. thanks for the link leo!