I made some more progress and a whole lot of research for any little detail I could find.
The sensor should be a Jaeger 3-wire non-amplified P/N 60526262. If I can get to the service garage where the car is I can pull the rear seat and see if there is an amp once and for all, but I'm quite sure no 99%. Why? I also have the speedo pulled from the car and it has the 6-wire connector plus the additional green wire separately. This is info from a post of Alfisto Steve who has a GTV6 speedo p/n 60701830 with exactly this characteristic. I have a different p/n since it is a kph speedo.
None of the wiring schematics, including that from the delivered car user manual, show wiring of such speedo and 3-wire sensor. It has only a speedo with 6 wire connector and a 2 wire sensor (where the wires are pink and white-red).
The Jaeger wires are black, grey, pink/black. The speedo internal wirings are red, yellow, black , and green. What I think is:
black - ground
red - +12V
green - signal from sensor
yellow - not sure
I don't have an oscilloscope on my bench, but if I wire the Jaeger as follows:
black - ground
grey - +12V
pink/black - assume signal
and I measure across grey and pink/black or +12 and the assumed signal, I can see a voltage variation when moving a metallic object near the hall sensor. That tells me at least that the sensor is not dead, whether it is performing to spec will have to wait for an oscilloscope.
I partially disassembled the speedo to see if there any macro issue. The only one I found was the the partial odometer was somewhat stuck, I couldn't reset it with the front button (felt like a worm gear problem) but I could reset with a plier and that may have loosened it up. This have could have been the root cause the speed to stop working....a mechanical issue. The circuit board looks fine visually, but would like to do other testing. Unfortunately I have yet to figure out exactly what the yellow lead is suppose to be. On the wiring diagram for a speedo without the green lead, the yellow lead on the connector is labeled as "1" whereas red is "+" so it would still seem to be sensor signal.
The plan is to see the sensor via oscilloscope and test the speedo with a signal generator, probably square wave once I understand the amplitude the sensor provides. The various markings on the speedo tells me the calibration frequency.