Prime,
nice looking truck. I have found that making a cheater wire with aligator clips on both ends is a great trouble shooting tool. This won't be the last time you will be tracing down wiring problems. I use this cheater to check for ground problems. I clip one end to a good ground on the truck and then contact the groung connection on the problem light. If it lights your sure it's the ground. I also use a test light to check source problems.
Welcome to the sickness!
THING 1 - 1973 88 SIII - SOLD
THING 2 -1974 88 SIII Daily Driver - SOLD
THING 3 - 1969 88 SIIA Bugeye Project
THING 4 - 1971 109 SIIA ExMod - SOLD
THING 5 - 1958 109 PU
THING 6 - 1954 86" HT