Greg Manning has a tried-and-tested method to ensure his trailer light plug stays waterproof and wired up
Boat trailer electrics, especially trailer light plugs are very susceptible to damage, either through the wires being pulled out or through corrosion. I have a sure way of avoiding this for both sockets and plugs.
First strip and tin (solder) the ends of the wires in the normal way and fit them into the socket/plug. Test to make sure all the lights work and then undo the clamshell of the plug or remove the socket from its base.
Fill the void around the terminals and the whole of the inside of the socket or plug including the wire entry port with silicone mastic.
Close the clamshell or remount the socket and then fill the grommet that goes round the wire with silicone mastic and slide it into position. Once the silicone has set no moisture can get in to corrode the terminals or wires and the wire will be so secure they won’t easily pull out. If the socket or plug gets damaged in some way, simply cut if off and fit a new one – no moisture should have found its way under the sheathing on the wires so they’ll be good as new and ready for re-tinning and connecting to a replacement plug/socket.
In 30 years I have never had a plug or socket fail having used this method.