Ed, I couldn't find a picture of a 986642 so I will assume it is similar to a 986250.
You probably know this already but I'll throw it out anyway. The contact-mercury-contact part of any mercury switch rarely fails. It's the parts going from the switch (one end going to the power source, then other end going to the bulb). A mercury switch can be physically broken. Mercury is toxic and you don't want any of it contacting your body.
I suspect the light doesn't work because the wire is broken and just like hair dryers and hand drills, 99% of the time it's where the wire enters the housing or enclosure. Unfortunately there is no strain reliever in that area so the wire is free to move around due to normal vibration of the truck. How do you get the housing apart to fix it? I don't know. Since there is mercury inside, I don't want to offer a guess.
The original switches bring big bucks on evilbay. I would post it for $50 plus shipping with a note that it doesn't work. With the proceeds I'd buy a modern switch that has a built-in strain reliever.
Last edited by buoymaker; 07/05/2021 4:19 PM.
