I've spent the day trying to figure out why the canopy 3rd brake light on the 1999 F150 I just bought is not working. I'm no expert on diagnosing electrical issues so I would really appreciate some help.

I replaced the bulbs in the light and tested it with an external power source and it works fine, but for some reason it is not powering on when connected to the truck. The power is supplied through the green wire that comes out of the harness at the back of the bed. It is grounded to the bed behind the driver's side brake light. I cleaned this ground and checked all of the connections.

Just for kicks, I tried connecting with that green wire and the ground from the passenger brake light harness and lo and behold, it powered on. What could this mean? Is there some problem with this ground and should I try to use a different ground? What's the difference between the ground on the bed and the brake light ground? I don't know what to do. I don't know if this system has worked before or if the previous owner could never get it to work.