One dim parking light?

Ground path for these bulbs begins where the metal base of the bulb meets the socket. If the inside of the bulb socket is crusty there may be only tiny contacts at the keeper on the bulb base. Spring tension and vibration would keep a current path there.
You didn't say what model so I'll add this... A pot metal fixture will have the bulb socket crimped into it. I've seen the ground path lost there. Previously someone had melted a lot of solder in around the crimp,,, didn't help. A tiny sheet metal screw though to connect both parts cured it.