Use a second relay from the running lights. If they are not on (i.e.
daytime) then the first relay (powered by the original backup lights)
will not function. This second relay carries virtually no current
other than the short interval activation current for the first relay.

                                                David(dsl82westy)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

>I agree with Joel on this one.  A small fog light makes a great back-up
>light.
>
>On thing that I do different though: I put the relay in the back close to
>the fog light (behind the tail light) and only run one wire to the switch on
>the dash.  I pull the power from the B+ post on the alternator (fused of
>course).  The switch on the dash completes the ground circuit side of the
>relay to switch on the fog light.  The up side to this is that even if the
>wire rubs through, nothing will short out.
>
>Oh yeah...  you should never, ever, use the fog light to light up that guy
>behind you with his high beams on.  That would just be wrong... ;)
>
>Cheers,
>
>Jeff