Speed-volume control for Mach 460?

I recently traded my '02 GT ragtop for an '01 Cobra convertible --#1761 of 3384. Both had the Mach 460 sound system, however the speed-volume function --pretty useful on a ragtop, doesn't work on the Cobra. The radio was switched out under warranty but speed-volume still doesn't work. I'm beginning to think, that while the radio can respond, speed-volume was not built in on the '01 Cobra, and hence there is no "speed" sensor input available to the radio. Can anyone help... Where does the system get its speed input? ...is there an after market sensor available? ...if, so from whom and how much?

It would be great to hear tunes again --without having to continuously adjust the volume!
 
There is a tech article that my friends and I wrote on this website that tells you how to hook it up.. Look in the articles section or in my sig below.

02's came from the factory wired up for it.

Shawn