Could also be that the hot wire is being shared with something else sucking all the juice from it.
If you have a multimeter or volt meter, remove the bulb from the socket and check voltage there.
When looking in the socket, if there is one metal circle in the bottom of the socket, that is the positive, the negative is the socket itself.
If there are two metal circles on the bottom, then one is positive and the other negative.
With the vehicle off, and just running lights (no head lights) you should be getting about 12 to 13 volts. With head lights on you could get anywhere from 11 to 13.
Let me know how this goes