I had that problem a month ago, noticed one of the lights were dimmer than the other, just thought the bulb was on it's last legs so just waited for the end of it. Finally I noticed it did but when I went to disconnect the plug for it, the plug had melted partially, probably because the connector in the plug was a little dirty or was barely making contact.
So check your plugs, clean them out real good, scrape the terminals on the bulb and then plug them back in and see if that makes a difference, GM replacement plug prices are a killer and the solution to replacing a plug is cut and splice, not plug and play!! :nonod:
Keep us informed!!