Fuse burns out on a W123 from 1976 when headlights are turned on

I suspect a wire - somewhere - has lost its insulation and is touching the body causing a short.

If the car was in a wreck look at the wrecked area for a bare wire. Otherwise, if the parking lights are OK its much simpler, for then the problem involves only the headlights vs. the whole lighting system. Then start at each headlight and follow its power wires back, back, and back toward the headlight switch. But if the parking lights blow the fuse .......

You need to use a short finder to find out where the wires are shorted.

I had this problem with my 81 300td, and finally found it was a bad high beam. Took some head scratching, but I finally isolated it there.

Hope this helps!

tweaks

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have you taken the light switch out to check the connections sounds like there is a bare wire somewhere touching earth