1997 GMC Sierra Headlight Switch

I have a 1997 GMC Sierra 4x4 Z71-the headlight switch has 3 positions -off, running lights and headlights. Recently the headlights would not come on when the headlight switch was turned to the on position. I replaced the headlight switch 3 days ago and now have the same problem. The brake lights, turn signals and running lights work when the switch is in the off position and the middle position but when turned to the on position the headlights go off. I have checked all the fuses on the dash and under the hood. I can not find any schematics for the headlight switch. Do you have any troubleshooting suggestions?