headlights dimming

While driving even at steady speeds my headlights dim. It doesn't matter if the high beams or low beams are on. I checked all grounds, and the plugs to the bulbs. Any ideas?
91 1500 auto 350 4x4.

Check the output of the alternator. I would take this to Autozone they will test the battery and the rest of the charging system for free......they can tell you weather the battery is bad or the alternator. Better to catch it before it leaves you sitting on the side of the road in the middle of the night with a dead battery and a toasted alternator.

There are several possible sources for this problem. It could be corroded or broken lamp sockets. It could be the voltage regulator in the alternator (previous poster gave good advice re: Autozone check) Last but not least make sure your main ground cable is not broken or damaged and the connection to the frame-and-or-block is secure. A good check for the regulator is to check to see if the headlight output changes with engine RPM. If it does then it's probably the regulator/alternator. If it's intermittent (lights get bright or dim for no reason) then it's probably an electrical connection.