Voltage problems, cutting out when hot

Ok, I removed the jumper between the yellow wires at the interlock relay (engine side) then I got under the dash and removed the interlock unit (green box) tested all connections at pins.
Put it all back together without the yellow wires jumped and re-tested.
Now - key off
0v at yellow wire at ignition harness to switch
0v at yellow wire at start relay
0v at brown wire at start relay


Here’s something I’ve completely forgotten about and have no idea what it would go to.
Key buzzer?
Horn relay?
Unused?

Red wire into plug has constant 12v (battery)