Corrosion on it. Scrape the contacts with a knife, apply dielectric grease to the ends of the bulb.
I'd pull the light up to and make sure corrosion hasn't broken the connections with the wires.
I had to repair mine after bumping them with my fist almost every time I needed the running lights to get them to work.
GW told me what the brand and model were, but I never found the exact model, so just cleaned up and greased the old ones.