Circuit/System Description
WARNING: This page is about a different car, the 2008 Cadillac XLR. However, it is still accessible from the selected car via links, so may be relevant.
The engine control module (ECM) creates the vehicle speed output signal by pulsing the circuit to ground at a rate of 4 KPPM. The VSS input is pulled high to B+ in the radio. The ECM monitors the voltage at the vehicle speed signal circuit to determine when the voltage is out of the normal operating range.