Welcome to 2CarPros.
Since it is only one light, I have a few questions. First, which side of the truck is it and which bulb. Next, you indicated it was narrowed down to the wiring harness. Is there power at the harness you are referring to? If there is but it doesn't make it to the bulb, then there is a break in that wire at some point. On the other hand, it could be a corroded connection. Have you tried to wire direct and bypass the plug? Also, I know that aftermarket light sockets have different color wiring. Are you sure it is connected correctly?
Let me know which side and bulb so I can study a few wiring schematics to better help. I will watch for your reply.
Take care,
Joe
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Thursday, August 15th, 2019 AT 7:14 PM