must be AFTER the relay--thats where power actually comes out from after fuse protection
grab the white wire anywhere from there to the back up bulbs- thats an easy place to access the wires,,the bulb~
12 volt test probe or multimeter for numerous elect tests is a smart buy for every toolbox
5 bucks at parts store, harbor freight gives $4 multi meter away all the time, ck coupons
congrats on investing in safety with a camera!