While installing the Empeg in my Miata, I again remembered that the oem radio had the ALC (automatic level control), where the volume would ramp up with speed. The speed is read from a pulsed 12v source (4 pulses per revolution). There are numerous ideas about speed controlled volume around on this board (not all including speed sensors but road noise):

my first thread
a newer one
an older one
and a very old one

I have the wire with the speed pulses right there next to the harness and could try any ideas, solder any provided diagram almost immediately. I see the road noise method as a very valid alternative, but once the speed sensor is input into the empeg, the possibilities just go beyond volume control
So whadaya say? What should I do with my poor lonely speed sensor wire, begging me to use it?