Radio Shack has some automotive relays and even has a harness that fits the relays. I used one on the Kodiak because I didn't want the full current going through the backup dash indicator. In this case, with a switch, you shouldn't really need one. If you could find a pushbutton switch to catch the shifter lever when you go to reverse, that would be the ticket. Problem is finding one that would be fairly waterproof. Have to see what I can dig up.....