Originally Posted by
waldig
If I had to do this I would use a 2 volt zener to block the voltage, this will drop the operating voltage by 2 volts also eliminating full power which could be an issue.
The simplest approach is to use a relay as your stand off, and then you would be able to get normal operation. The relay could be a small 12 volt single pole relay and you would not need any additional wiring. See my sketch below.
I have made a sketch of the relay, and a wiring diagram. When the relay sees about 8-10 volts it will PULL-IN and power the output pin C. Should do all you need, relay to be as small as you have, no current to speak of here.
HO HO Woody
Only issue would be that the backup light bulb could blow and the bulb sensor stuff would never know ( not much of a concern, though - who needs 2 backup lights anyway ? )