Willie, I don't believe there is a switch for it but there is a wire from the speed pulse line that goes into the radio to allow the radio to do this.

The effect is rather subtle, not like the Delco units on the Corvettes and other GM cars that seem to want to blow your ears out or not work depending on which setting you use.

If it is not working, it could be that the speed pulse line is not properly connected. It is subtle and I didn't notice it when I test drove the vehicle.