Took the switch out tonight. Was examining the backside and found this:
See it? Bottom right. When in neutral or left turn position, the wires are touching and the headlights work:
But push up on the stick to turn right, and the cable in the arm is pulled ever so slightly and the wires disconnect. That little bit of flexing after 205K miles of changing lanes under boost /ubbthreads/images/graemlins/smirk.gif eventually caused it to break:
The headlight relay is grounded via this wire by the headlight switch. So I stripped and cleaned the ends, soldered them together and taped the connection back up. All good. Here is the circuit with the junction marked:
