[flybouy]

Sounds like you are going in the right direction. Have you checked the lights sockets for corrosion? A green discoloration on the sockets is a common issue. I'd suggest a longwire that you have grounded to the trailer. Then you could check continuity between the the known good ground and the lights ground. If that's good, then check for voltage from the lights positive wire to ground. Where the clearance lights are grounded could be quite the treasure hunt. Hope this helps.