Vehicle Speed Sensor (VSS)
In addition to the standard power and ground leads, remote-mount navigation (and even some in-dash units) systems connect to a vehicle's Vehicle Speed Sensor (VSS) wire. Your vehicle's cruise control uses the VSS wire to determine the vehicle's speed — it serves the same function for the navigation system. (Don't substitute the tachometer signal wire for the VSS wire — the tach signal wire sends pulses as long as the engine is running, even when the car is sitting still.) The VSS wire sends a series of pulses (from 800 to just over 1,000 per mile) only when the vehicle is in motion. If the VSS wire is not hooked up, the navigation system will not realize that the car is moving at all, and will be unable to track position.
I'm not too familiar with this wire or the installation of navigation systems. I found this link though and it seems like it's what you are looking for...
"Speed Sense WHITE/ORANGE (AC) IN HARNESS ON INTERIOR DRIVERS SIDE OF FIREWALL"