I would like to know what signal the remote volume control sends to the head unit. I want to design a device to automatically adjust the volume level based on interior noise levels.

 

This is straight from S2Ki.com:

The dash controls (with the exception of the MUTE button) work by varying the resistance from ground (pin 20) to pin 3 of the head unit. The MUTE circuit is slightly more complex because the button is a momentary action type (it makes and immediately breaks the circuit) and a latch (or hold) circuit is required to keep the MUTE input to the radio (pin 14), active, and light the LED on the switch.

For those interested in making their own adapter you'll need the following information on the resistances produced by the controls (all measured between the radio connector remote input, pin 3 and ground, pin 20). Some cars may vary slightly.