Brake/stop light problems

[grog17]

I have an '01 Cevy Express that has functioning brake lights...Until I turn on the headlights..The lower bulbs go out and only the third brake light comes on. I checked/replaced bulbs., cleaned sockets/ground/ connectors, all with no luck. I was told all the lighting is routed through the turn signal stalk. Could this be suspect?

[cokedrvrbjw]

How in the hell did you see that?

[GM-Tech]

The turn/stop lights go through the turn signal switch... except for the CHMSL (third brake light) which gets its power directly from the stop lamp switch.

 

I would suspect a ground issue.