Pull the bulb socket put of the housing and pull the bulb out. There is a 99% chance it is corroded to all hell. Condensation collects in there. Go to autozone and buy PN 85881. It's a generic 3 wire GM light socket. Usually Dorman or the like. It's $8. The GM socket is $30. If yours isn't bad you can get the inner piece to pop out and swap those very quickly. If you can't you need to cut and solder the 3 wires.
RTV the socket where the wires go in, pack the socket with dielectric grease, and drill a small drain hole in the bottom of the signal housing at the back corner. It should last a couple more years. You might have to do both sides to get it working right.