What kind of connector do you have? IIRC, the 7 round rv style, the center pin is for reverse (I am at sea and do not have my RV book). You could tap off the reverse light as Jeff said and connect it to the connector, but depending on what kind of lights you are using on the trailer as reverse lights (I had 55w halogen driving lights that I used to be able to see when backing into spots at night), it may draw to much current. you would have to tie the tap off the reverse light on the truck to a relay and use an alternate power source to power the trailer lights. I just tapped the reverse lights on my camper to the house battery on the trailer and put a toggle switch inside my fridge access panel... stop, flip on the lights and then back into the campsite.
