Limbhook

If the tail light was a sealed unit and the bulb had water in it the seal was broken. You'll need to replace the tail light. I agree with locator if there is any age to your trailer replace all the lights. One thing if you replace the lights and don't have a fish tape ( that's Not for measuring crappie) attach the new wires to the old wires so you can the new ones in as you remove the old ones. Another way is to attach a string or wire to the old wires before you pull them.

On your earlier comment about the ground. If the system was not grounded none of the lights would work.

Hope you get sorted out and are fishing Saturday. Something else you could try is hand signals. Typically people wave when you use them because they think your waving at them since no one knows what they mean anymore. On second thought protect your motor, get your lights fixed.