:Back-up lights

My 66 valiant "Victoria" does the same thing, When I first got her I found this out by parking it in reverse, It is a three on the tree and walked away from the car and seen my reverse lights on. Makes you wonder if it was wired that way from factory she has 57.000 miles on here and my wiring has never been touched that I know of. But I did change out to electronic ignition