1999 Blazer 4.3 Vortec headlights don't work all the time. At night when they are automatically supposed to come on, they don't. You can't even turn them on manually with the switch when this happens. You can go several days without problems then all of a sudden they wont work for several days. Replaced the relay for them and it made no difference. Doesn't seem to be a short, hitting bumps in the road and wiggling wires makes no difference. A while back though I did hear some buzzing under the dash but could never locate it. Is there any other component in the headlight wiring that could be checked besides the fuse inside and the relay under the hood?

 

***UPDATE*** I broke out the multimeter and was checking the wires leading up to the headlight bulbs for continuity today and discovered that both leads on the plugs are energised (positive) when the lights aren't working! WTF? These are plastic mounted headlights so I know for a fact that one of them are supposed to be grounded. So now I know why they aren't working but what I don't get is how the negative can become positive!

 

youll need to find a schematic and start digging, i believe these newer trucks 98+ are a switched ground headlight system so you may have a bad ground which would lead to seeing positive on both sides