If I were you, I'd start by tearing apart the 4 pin connectors for the blinker + headlight (assuming it's like the W123)
Look for shorted out connections.
Look for naked wires.. I had a weird blinker/reverse/etc problem and turned out to be a few naked wires in the trunk touching eacc other.. I guess these naked wires get too excited..
Work your way back from the connector (trace the wires)
Have u checked the fuses by any chance?