All of our exterior lights were working up until last trip. Hooked up, and on our pre-trip check, the running lights and one brake light were not working. Fixed the brake light by replacing a blown fuse. Went ahead with the trip, as it was only a couple of hours away and daytime travel. Now that we're home, trying to figure out the problem with the running lights. Brake lights and turn signals work fine. We plugged it in to SIL's truck at camp site to test and same problem, so it's a problem on the trailer, as opposed to the tow vehicle. Any advice on where to start?