a quick and dirty solution would be to (if you still have them) take the main board out of the original clocks and sneak it under the scuttle to provide the signal to the ECU. You should just be able to provide the board with some power to get this working. At least then if you split the signal between the new speedo and the old board, you'll know that what is being passed to the ECU is the correct signal (that may not be the same form or voltage as that from the diff). It would be cheaper than getting the ecu reprogrammed.
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