Cheeze,
Wiring the old fog lites into the back up lite wiring would be the easiest but (1) it might cause too much current draw thru that original 16 or 18 gauge factory wiring harness and (2) there are times when you might not want them on while backing up. It might confuse people behind.
Wired on a heavier aux circuit you could use it as a work lite and a selective back up lite for campgrounds.
In the 70's a sailor in my squadron in Norfolk use to go home every weekend to NY or NJ and got tired of tailgaters behind his van on old two lane I-95 and wired in an aircraft landing light facing aft. He solved his tailgating problems until he lit up a state trooper and the trooper turned on his lites and siren. I don't think he got a ticket (some LEOs give military a break) but I think he had to remove it before the trooper would let him go on.