Well I'm sure you swapped a known good bulb to the side that didn't work and checked the suspect bulb in the side that did work.
If you have access to a multimeter and know how to use one check for voltage at the plug where the bulb goes in with the lights on. Check for ground as well...lights on not needed. IF you are missing either of them, light no worky.
Also look at the plug/socket where the bulb plugs in. Corrosion in the socket could kill the light. Also look at the wires going into the plug. Make sure they are connected securely. Give em a tug where the wires go into the socket. If that fails, maybe a trip to the dealer.