2000 cavalier headlight problems

We have a 2000 cavalier with a headlight problem. The daytime running lights work, but when we turn on the headlights the left one doesn't work. Or is very dim. If you pull back and hold the dimmer switch it works. Any ideas?
Thanks. You might try swapping the left and right bulbs. That will tell you if a bulb is bad, in which case you will replace it. If both bulbs are OK, look for a weak or corroded connection at the socket or ground.