67 XR7 Mystery wire

A grommet from a hole located at the drivers side, near the brake booster, that feeds the washer fluid and a vacuum line? through the firewall was missing. In trying to get the right size grommet I found this wire with connector just sitting in the same hole. The wire feeds from the main harness on the engine bay. The car hasn’t been started since last fall, but as far as I can tell most of the electrics work. No idea what it could be and can’t find any obvious loose wires under the dash.

Thanks for any help
Cheers
Jack

Plugs into the brake warning switch, that is screwed into the brake distribution block.

Thanks! That makes sense. When the fronts were converted to discs an after market block was installed.

Cheers
Jack