The onstar GPS reciever is on the roof on the outside usually rear passenger side.the Gps that earlier nav. equiped models had the GPS was under the windshied defrost vent.
If you'd like google "hacked Onstar" if you don't plan on using the service you can solder a usb/serial cable to the onstar control module and use a PDA/PHONE or Laptop to navigate youhttp://www.openminds.net/onstar/1explanation.html