If there is a GPS antenna in that unit, then there is a GPS receiver somewhere.
Basically, you'd need to get the signal outputting NMEA data to a serial or USB cable. considering that several members have taken OEM GPS modules with an antenna and used them with PCs, I think this is something that's very doable. I also recall someone hacking onstar, but I don't remember if it was specifically to get GPS signal or not. Search the forums and see what you can find.
Have you looked in the FAQ yet?
How about the Wiki?
Under normal circumstances, a signature would go here.