The only way I know is an item called a Lockpick and it is a harness that plugs into the factory headunit and enables all the items that are restricted while driving. I know that in most states having a video playing from the front end of the vehicle is illegal therefore factories disable that feature all together. On aftermarket HU the work around maybe be easier due to the fact that you can trick the HU into thinking the vehicle is in park by attaching a line from the HU to the parking brake. From what I can gather its not that simple to do with Factory HU.
The Lockpick is around $200. I know they have different ones that have added features therefore the price goes up. Just google GM Lockpick and you will find it.