What's this wire and grommet for?

[69f6bee]

There is a wire coming through a grommet that is located directly above the starter relay, between the main wiring bulkhead and the master cylinder backing plate. My 69 super bee doesn't even have a hole there, but I see this on several other b bodies.

 

[440SportSatellite]

I would venture a guess that it's the tach wire.

 

[Meep-Meep]

Yep, sounds like the optional tach wire.