I figured it out, for anyone who is interested. Use a standard 5-lead Bosch-style automotive SPDT relay.
85 ground
86 is the turn signal wire
30 is 12V from the battery
87a is positive fog wire
I think that would work to turn off the fog light when the blinker is on, or to dim the brightness maybe just put a resistor? I don't know if that would be safe.
edit: oops I believe it would be 87 not 87a, not sure haven't test it yet.