Unknown wire in engine bulkhead connector ???

[Green Machine]

Heres a pic of the engine connector on our 1971 C20. Has 350/350 no AC.

Redoing harness for HEI, electric choke, and tach.

Middle top wire - green - hard to see in pic is temp sender.
Top left light brown wire goes to brake prop valve
Big yellow wire goes to inline fuse and then to HEI (I replaced resitor wire)

Bottom left dark bown wire is the unknown wire, it goes into the wire channel and then is terminated with a plastic male end and hidden inside the plastic wire run. I checked the fuse box side and it is connected and dissappers into the large bundlel of wires going up under the dash. So in must or could have a purpose.

Middle botttom purple wire goes to the starter.
Bottom right blue wire I added for tach connecton (was unused)
(added matching connector to fuse block also)

Does anyone know what that brown wire in the left botttom corner is for ??
I could remove it to clean up the harness a bit, but it is connected on the other side, kind of hate to remove it if it is good for some thing, even if not used now - could that be a factory tach wire ? - truck does not have factory tach.
Thanks for your help.