spdt relay output converting polarity

What is the output of terminal 87 in the diagram below if terminal 85 is engergized (12V+)? I need to trigger a 12V+ output when an existing wire (backup lights 99 Ford Windstar) is switched from 12V+ to ground. Thanks in advance!

What exactly are you trying to do?

New head-unit has a built-in screen and 12v+ switch to automatically display video from a backup camera when the vehicle is placed in reverse. Normally I would just tag into the reverse lamp at the tail-light so that when the light comes on, that sends 12V+ to the head-unit switch, but unfortunately the reverse lamps on my 99 windstar don't work that way... both inputs to the lamp are powered until the vehicle is put into reverse and then one side is grounded which turns the backup-lights on. So basically, when that wire switches from 12v+ to ground, I need a 12V+ signal to be sent to the head-unit.

Thanks! I'll give it a shot.

Craig, your comment about the Ford wiring reminds me of working on an S type Jag a few months ago at a friend's garage a few months ago Right hand indicator not working and the bulb was OK so I blurted out "bad ground". Oh just meter it says he and sure enough we find it's a bloody Lincoln town car made in England! Rerouted the constant power supply to that bulb and job done. Just one thought, as you said why? Anyway it wasn't a bad ground it was bad live!