Hmmm--I'll try this again. Forgive me if this is a double post.
The best way to approach your problem, FF, is to replace the wiring in your trailer. By re-wiring the thing, you can get the new waterproof lights, which, in five years of use in saltwater, have yet to fail me. I trailer a lot and have to sink the lights each time I launch and retrieve my boat. Re-wiring takes maybe an hour for an 18-foot trailer, and when you do it, you get all new connections and no corrosion.
Another option is to build a light bar-- something I did way back when I had multiple boat trailers and was constantly fighting the lights battle. I used 2x2 lumber to build a bar that rested across the back of the boat on top of the gunwales. It took a bit of tinkering to ensure that it would'nt roll over or slide off, but it wasn't a big deal. I fastened the bar with bungee cords to the boat and installed the lights on top of the bar. The wiring harness was then taped together and draped over the boat to the connector.
When launching, I simply unplugged the bar, rolled the wire up and placed the bar into the boat. In about 10 years of use, I never had a failure. However, the new waterproof lamps (I have them on three boat trailers) have yet to fail me.