x2 on grounding issue. use a multimeter from the positive side of the socket to ground. If you get a full 12v start working your way back from the socket on the ground side testing ground along the way
With a suspected bad ground, a multimeter will show 12v, but a load will have high resistance with the bad ground and have a voltage drop.
Clip a jumper wire on to the suspected bad ground and the other end to a good ground.
Maybe what you were trying to say is check voltage from the tail light metal bucket to ground and if you have anything over 0 volts you have a ground problem