I have a 1983 240 Diesel. I am having an issue with the headlights.

The ones on the driver side don't work. The passenger side ones do however. I have replaced the headlight switch, headlight relay, and the headlights themselves, and nothing has worked. Keep in mind the side lights (also known as parking lights) work on both sides. I cannot figure out why the headlights on the one side cease to function. Anyone know any possible causes of this?

 

Sounds like the bulb failure relay.

Same thing on my car only it was the driver's side tail lights or brake lights that wouldn't work.

You can take it out (big red round one) pop off the cover and try re-soldering the solders. It won't always look bad, but some of them have microscopic cracks and re-flowing the solder helps. Also make sure the connections are good before you go opening this thing.

The cover pops off easily with a small flat tip screwdriver, a precision one.