blinker stops working when headlights come on
This is a very repeatable problem. When the headlights come on, the left
front blinker stops working. I've been able to reproduce it in my garage. If it's
dark in the garage, the headlights come on and the left blinker stops working.
If I shine a light on the dashboard sensor, the headlights (eventually) turn off
and the left blinker starts working again. This is without touching anything
else, thus I don't think it's a relay, blinker module, light bulb, or column switch.
What's going on here?