In sci.electronics.repair micky wrote:

And what is it equalizing?

A number of car stereos have a setup that is used when first installed to
automatically set the volume and eq (bass/treble) for each speaker so it is
optimized.

When in that mode, the radio plays a number of tones and white noise one
speaker at a time to figure out the best pattern for the driver. That
microphone is used to listen to those tones. You aren't in the car, you
place the mic where your head would be and get out of the car and let the
radio run the tests.

You don't leave it in circuit, after the radio does it's thing, it's
removed.

Both my original radio and the current one has modes for "driver only",
"front only" and "4 passenger". It's just a tweak to get the best sound
based on how many people are in the car.

Unless you have a radio that does the auto configuration, there is no point
having one of those mics.

-bruce