Make sure you have good grounds in the electrical system.
I have a good 6 volt battery that passes the load test. I was showing that (6 volts) across the battery but at the end of the positive cable it was showing 2-3 volts. I thought it was the ground at the engine. I was wrong it was the battery ground.
Lesson learned
On a 55 2nd I used to have I could not get the tail lights to work properly. I found grounding the bed to the frame was the problem.
Make sure you always have good grounds.
Best of luck
