Before shows the basic parking brake which is a switch to ground when the parking brake is engaged.
After shows the modification to add a DIODE, in my case an LED (Light Emitting Diode) to allow current from the parking brake sensor in the head unit to flow through the actual parking brake. And a secondary path through a switch that will allow a simple SPTS (Single Pole Single Throw) to allow the radio to detect a ground / parking brake when the real parking brake is NOT set. The diode prevents the switched ground signal to be isolated to the radio head unit but not affect the car.
The switch is not shown in the real-life picture but closes the circuit between the green parking brake sensor/red LED lead and the gray wire to ground.
When the real parking brake is set, the LED will light as it provides a path to ground. When the fake switch is closed, it will NOT light.
The diode part of the LED blocks the signal from the car from flowing through the secondary switch.
Before.png

After.png

17ParkingBrakeLED.JPG