I have successfully wired in my relayed Cibi� Airport fog lamps on the
front of my Vanagon.

Now I am contemplating my options for a Hella wide lamp out the rear. I am
afraid of creating a situation where the bright rear light could be turned
on on the road accidentally, potentially blinding drivers behind me.

The simple solution, which is not all that simple in practice, is to wire
it into the reverse switch somehow.

What about this: put a regular relay into the backup circuit, where the
trigger wire is the hot wire from the reverse switch and opposite that is
the ground. Rather than have the main 12v source to the light running
though the other two terminals, these terminals would take a trigger from
the dash switch, which would operate r a second relay. The power circuit of
that relay would go from the battery, through the relay,  to the light.

This way, if the dash switch for the bright reverse was on, but the first
relay received no trigger from the transmission switch, the bright backup
lamp would not come on. If the car was in reverse, and the dash switch was
on, the two relays would complete the circuit and supply strong 12v power
to the light.

??

Jim