Ifyou're using a standard 4-flat connection, see if the brown wire goes hot (+12vdc) with respect to groundwhen you have the taillights on.
It should be 0vdc to ground when the lights are off.

How did you connect the car/truck to the plug?
Using a 'T' connector that just plugs into the existing wiring harness, or did you have to chop into the truck's wires individually?


Thislink is a pretty good resource too...
http://www.etrailer.com/faq_wiring.aspx