89 Chevy S-10, Pickup. My Tail, brake, and signal lights do not work when the Headlights are on, Is it definitly the switch or could it be the build up of Ice forming on the wiring harness near the rear of the truck where the wires seperate .

IT sounds like it is a short in the electrical system. maybe

are the turn singal ind. on in the dash cluster lit when the head lights are on?, if so you have an open ground circuit in the lighting harness. let us know.

The turn sig' slows down on dash cluster, but doesn't shut off? However the weather warmed up in the mean time and everything has started working again except the license plate lights which are most likely blown from the abuse of being cleaned this winter.Thanks for the Help by the way!

Check your ground I had a 1989 chevy 1500 did the same thing. Ground wire was hooked to bed just about in the middle where hitch went, it had come loose and every once in a while lights wouldn't work.

I thank You all for your submissions! I think I have figured it out and it was a broken wire, most likely a Ground!!! So now I just have to fix the License plate light and that too is a broken wire, do to the rather large build up of Ice and "Road Crap" built up in the driver side rear tail light area where the wiring harness splits to go to the other side of the vehicle. Was kicking off the excess "Crap" one day and noticed that there were wires ensnared in the "Crap Chunk" So anyway, the next problem is even worse! Truck runs like it is starving for fuel, but need to do the obvious fixes first, Like Filters etc... before bothering You folks for help! Thanks again and Ciao for now, Grambo40