One other hint.

Corrosion is the bain of all trailers. This is both within the
connectors and the lights.

I found that if I coat all connections with a "dielectric grease" I
never have any problems even after the trailer has been outside all
winter and the car has been dragged through salt spray. This is
basically a very good petroleum grease. I coat both the male/female
connections between the car/trailer and inside the lights between the
bulb and the contacts. Dow Corning mades a good product for $8 a tube
or so which goes a long way. I have found it at automotive shops,
trailer shops and online. Steer away from the cheap black stuff as it
makes a mess.

I also have found rubber boots for the trailer/car connectors. This
further protects this vunerable connection. I have found these at all
the big box stores (Wal-Mart, etc).

Good luck and let me know if this helps.

Johnatderosaweb.com