I have ordered a replacement head unit, Pioneer AVH-X7500BT, that has a detector for the parking brake. I want to install it with a tap into the regular parking brake AND a fake parking brake that I can activate with a simple switch to ground. I think I need a diode in the wiring setup but how do I wire it in so that I can optionally fool the stereo to think that the parking brake is on, for video playback but not confuse the car to think that the parking brake is on.

Goal - car always correctly understands if the real parking brake is engaged but not the fake-out switch. Radio head-unit correctly detects if the parking brake is engaged OR if a secondary switch to ground is engaged.