Thanks for the advice guys.
I have a multi-meter, but don't know much how to use it.
What is a 'good reading' and what is bad?
My truck has no fuse box or fuses.
I have replaced the tail bulbs with a set of LEDs I had in an old trailer - tested and working.
There was a broken wire under there I found & re-connected but it didn't do the trick.
As far as throwing money at the problem - How much does a switch cost?
Last question I swear - Is there a way to modernize this with a modern switch and a new bracket? any value in this?