Chevrolet Repair: 1987 S10 Pickup Truck, headlight wiring harness, s10 pickup

Question
What would cause an 87interview S10 pickup to stop running when you turn on headlights, blow the horn or turn on the heater's fan. Headlights will not come on at all.

Answer
HI BEVERLY, THE DEVIL !!!! SOUNDS LIKE THERE IS A SHORT IN THE WIRING SOMEWHERE. I WILL ASSUME THE TRUCK WILL RUN OK IF YOU DO NOT TURN ON THE HEADLIGHTS, CORRECT ??? IF SO WE CAN RULE THE ALTERNATOR OUT OF THE PROBLEM..I THINK YOU WILL FIND A SHORT IN THE HEADLIGHT WIRING HARNESS. LOOK UNDER THE HOOD FOR THE MAIN WIRING HARNESS THAT FEEDS THE HEADLIGHTS..START AT THE HEADLIGHTS AND WORK BACK FROM THERE,SEE IF YOU SEE ANY WIRING THAT IS PINCHED OFF CORRODED OR JUST DOES NOT LOOK GOOD. IF YOU DO NOT SEE ANYTHING OUT UNDER THE HOOD LOOK WHERE THE WIRING GOES THROUGH THE FIREWALL [UNDER DASH] LOOK FOR THE SAME THING AS I SAID ABOVE. IT SOUNDS LIKE YOU MAY HAVE THE PROBLEM AROUND THE HEADLIGHT SWITCH OR IN THE AREA OF THIS BECAUSE IT IS TURNING ON THE BLOWER MOTOR CIRCUIT WHEN IT SHORTS OUT. AS FAR AS THE ENG STOPPING IS BECAUSE THIS SHORT IS DRAWING TO MUCH CURRENT FOR THE SYSTEM TO HANDLE..CK TO SEE IF THE HEADLIGHTS TURN ON WHEN THE TRUCK IS OFF. CK FOR A DIM LIGHT AND GO AFTER THAT WIRING HARNESS. THIS MAY TAKE SOME TIME TO DIAGNOSE. LET ME KNOW MORE AS YOU GO ON..GOOD LUCK,,,TOM.