Well i can point you in the right direction as i am a professional car audio installer. On front seat dvd packages by law wea re required to ground the dvd to a parking brake so that when the parking brake is engaged you can watch the dvd player but when you take it off it will only dispaly a screen saver or w/e your dvd shows. Now all we do if we wanted to bypass it is to ground that wire anywhere and viola you can watch whole driving. If i had a wire schematic for the dvd/nav system i could probably pinpoint GMs wire they use to ground out. My guess would probably be like on the nuetral start safety switch or transmission because thats most likely where your sensor is to send the out of park signal.