In my case the bulb was good. I did continuity checking with a VOM and found that the common contact in the socket was not making electrical contact with the side of the bulb. I cleaned the bulb and the socket contacts (socket bulb goes in). I found little corrosion or gunk. I also checked the plug between the socket and cable. The contacts looked a little worse here, but cleaning did not help. I carefully pulled the socket contacts out so that they would have a little more spring pressure against the bulb. Pulling the side contact out solved the problem (for now). I am guessing that the problem with these things is more due to metal fatigue on the side contact reducing the contact pressure rather than dirt/corrosion. If that is true, the problem will be back.