Name that wire

[Called3rdstrike]

Inherited a 1971 Plymouth Duster 318 2bc. Ignition was upgraded to an electronic ignition. Besides that all original. Found this stray wire hanging near the starter. Has a push on connector and is a black wire. Rest of the bundle goes to the starter relay. Engine runs and starts. Is this a left over from the conversion or does it have a home. Any hints on what to check is helpful. Found nothing near it missing a wire I can see.

 

[Mr Hemi]

It goes on to the brake valve. turn on the key and ground it and the brake light on the dash should light