F350 Lariet V-10 4x4 SRW automatic
 

I have unhooked the harness at the top of the rear end and this doesnt disable the speedometer, it only causes the white colored anti-lock brake light to come on and stay on, on the dash. Does anyone know exactly where the signal comes from on this particular truck that causes the speedometer needle to move? My neighbor tells me he cant do his testing without hooking up to the actual signal source. So, Im looking to find the actual sensor or mechanical drive that sends a signal to the electronic ignition and then on to the speedometer itself. Thanks