I thought in dash monitors had a wire that was supposed to be connected to the handbrake

The reverse gear seems more logical, as they've fitted it to my motorhome, because it already has a sensor/micro switch.
to operate the reversing lights - not that most handbrakes don't also have a sensor.

sorry I've just re-read yours, I see that you are not talking about a reversing camera - the guy from my car radio fitters delivered me back to the office with "Star Wars" on the dashboard, and he certainly wasn't doing less than five miles an hour!

I found this whilst looking for something more definitive.