At 07:54 PM 7/31/2013, Jim Felder wrote:
>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.

Use one relay.  Take the 86 feed from the backup light switched
circuit.  Run the 85 terminal up forward and use your selector switch
to ground it when you want the extra light to activate with the regular ones.

Yours,
David