Try the spray cleaner as mentioned above. If that doesn't work then take a small flat blade screwdriver or knife blade and scrape both sides of the metal tangs in each slot on the trailer plug, scraping until you get some shiny metal. That oughta' do it!
TV1: Black Sheep, 2001 Dodge 3500, 800,xxx miles, a few non-stock parts here and there...
TV2: Brownie, 1989 Dodge W250, only 256,000 miles!
TV3: 2004.5 Dodge 3500, 415,000 miles, 6 spd
TV4: 2005 Dodge 3500, 478,000 miles, 4spd auto
"Too much of what I once knew I now know not" Me