Ninety percent of the electrical problems on our MG's are from bad ground connections. Sometime lights work, sometimes they don't. If you find power at the socket and the bulb is good, then chances are real good that the problem is just a bad ground. To make things worse, it seems that some of the light fixtures were just designed to fail. Ever notice how some fixtures have a black ground wire attached to the socket and some don't? The one's that do not have this wire already are counting on a good mechanical connection between the ground side of the socket and the body part to complete the circuit and could benefit from a bit better connection.
Here's what I do:
Along the same lines, you ever installed an instrument in your dash and then realized that your light for that instrument does not work? Then you remember that damned black wire with the loop connecter that you were supposed to put under the thumbscrew but forgot because you were doing everything you could just to hold the instrument in place, hold the mounting bracket in place as well, get the thumbscrew started without dropping it into the behind the dash bottomless pit of wires and dark places that have not been seen by human eyes in thirty years?
Well, here's what I did for the instruments in my car.