You might consider the new LED trailer lights that are available now.  They are supposedly completeley sealed and should not have any corrosion at the contacts (since there aren't any) and are supposed to last the life of the trailer.  One type is made by SeaSense (www.seasense.com) and is sold by a number of vendors on the web; just do a search on "led trailer lights".  I just bought a kit from Dick's Sporting Goods for about $50 but have not installed it yet so I can't testify as to how well it works.  However, this technology promises to be a real breakthrough in the boat trailer light battle.

Steve Martin, M-15 "Cadenza"

> And then there are the submersible lights. These lights have an air pocket around the lights that is >suppose to keep the mechanism dry.