TEMPERATURE OUSIDE WAS ABOUT 100 DEGREES ON I-80 EAST OF SALT LAKE CITY CLIMBING TOWARD PARLEY'S SUMMIT. ENGINE WAS RUNNING A LITTLE BIT WARM AS USUAL WHEN CLIMBING LONG STEEP GRADES, BUT WAS NOT OVERHEATING. SUDDENLY, I SMELLED SMOKE AND THE ENGINE STARTED TO RUN ROUGHLY. I PULLED OVER AND OPENED THE HOOD WHERE THE WIRING HARNESS AT THE BASE OF THE WINDSHIELD WAS IN FLAME. I HAD A FIRE EXTINGUISHER WITH WHICH I PUT OUT THE FIRE. TRUCK IS STILL BEING REPAIRED, $5000 AND COUNTING.