Saw this once on a 54 1/2 ton. New buckets assembled from the vendor. Problem was the new 4 position strips on the fender. One is high, One is low, One is running light, one is turn signal. Ground is a screw into the inner fender. He had put the ground on the strip and it made a lot of funny stuff happen. Ground is to the inner fender. Bolts to the cab and a strap to the starter mount bolt, and a strap to the negative on the battery from the mount bolt on the stomp starter. pretty straight forward. I did not catch the ground on the strip for a little while, it looked OK so my eyes slid right over it. Standard 3 prong plug is from the back Hi on the left, ground in the middle top, and low on the right. I think, anyway there should be no volts back out from a light that is working. All voltage should drop across the working filament to ground. 8.5 volts will light up the dash light, just sorta dim, like what you have. So start with the wires from the lights. one wire at a time.
