84 300TD indicator lights come on when lights on

[wessie]

when headlights are off indicator lights work fine. When headlights are on (day or driving) it also powers on the passenger side indicator lights front and rear, and quite bright. Also the indicator function does not work anymore for that side. So, somewhere they are getting quite some power.
Could this be from fuse box? Could this be from light switch?

Any help please.

[Pooka]

It sounds like a headlight switch. These are quite robust items, but when the contacts inside start to break down then weird stuff like this can take place.

The easiest thing to do is to pull the switch, which is not hard, and test it with a voltmeter. I don't have the contact map, but I am sure if you asked for it someone could tell you which is the 12V in and what all the other contacts should be for which function.

And if you need to replace it almost any Benz from that era uses the same switch.

[vandor]

I've seen dual filament bulbs do this, when one filament broke and touched the other one. Also bad grounds at the lights themselves have caused some pretty bizarre symptoms.

Csaba