Hello I'm Scott,
It sounds like you have quite the dilemma here. I have a question, has this vehicle been in a wreck? Another, do you have a volt meter to do some tests for me?
Please be sure you are 100% satisfied with my answer before you hit accept, bonuses and positive feedback is always appreciated! In other words do not hit accept until we have your problem solved!
Thank you for that info. It almost sounds like a bad ground, so what I'd like to do is have you first ground your meter and test the socket to see if you have a full 12 volts going to it. Then if you have 12 volts I'd like to have you stick your meter on the ground side of the socket and see if we have a good ground. Does this make sense?
Right on, let me know what you find. One more thing just in case you haven't done so, try another bulb. I have seen faulty bulbs that don't ground properly. Just throwing some ideas here. Good luck here.