headlight issue

2003 Sierra: Last night I used my bright lights and then switched back to dim...but they didn't come on (yes I was "in the dark"). I went back to brights and they were okay. When I got to a place to stop, I turned the truck off and back on and no dims (only brights). The third try I got the dims back. WHile driving after that I noticed the dims flicked off and back on very quickly. Any help would be great.

I had a similar problem but my truck is a 91, my problem turned out to be the switch, I had no lights at all. the relay would be a good start point
When those lights go out at 50 mph on a twisty mountain road it sure makes your backside pucker real fast.
If you can re'create the problem get a light testor and start probing,