Typically, this problem arises from a corroded socket. My brother-in-law had an eclipse with this problem. Turned out to be the socket had corroded a bit, not allowing full voltage to the bulb. His also started to melt the socket from the high temp created by the resistance in the wiring. Check for darkened or melted areas on the socket, and replace if this is the problem. I used butt-splice connectors with built in heat shrink and sealant, and put dielectric grease in the socket to protect it. Never had another issue and the light was nice and bright. Then had to do the other side a few months later, lol.
If the socket looks good, check it with a multimeter for full 12V anyway. It could also be from a bad splice elsewhere, but the socket is more likely.