It's not that bad guys. First you just want to watch video, right? That only involves the parking brake to be grounded. With the brake wire grounded the car thinks the brake is always engaged, so the only need for a switch here would be to remove the parking brake warning light on your dash, when you don't want the video on. (if the light doesn't bother you, then no need for a switch here)
The only time a switch needs to be added is if you want to hack the nav unit in order to get around the car greying out certain functions on the GPS while driving. When driving it grey certain sections out so you are not playing with it while driving. This hack slips into the VSS wire and when the switch is turned off, this tricks the car to think it's not moving (the GPS does still technically work however it's just not as accurate) With the switch on the GPS works as normal, but the grey parts are back on the nav.
Hope this clears things up a bit for you!
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