my system works but it is only using the sky satellites so it is always slow to keep up if I drive in the city. I rang Alpine UK with the model and year who told me that the Canbus installation is required for an accurate speed pulse.
It links into the car wiring and extracts a digital signal converting it to analogue so the sat nav unit can pick it up.

Have a read of this supplier http://www.huntertransducers.com/