Unfortunately, I too have the same issue with vehicle speed signals.
The short answer is that there is no easy way to do this with simple wire splicing.
The PCM uses a frequency generated signal to measure inductive pulses through a "tone wheel" inside a GM transmission (usually on the output shaft). If you try to splice into an existing ABS sensor wires, you are now inducing a second frequency into a sensor that is already getting excited by the ABS module. Neither module will be able to read the sensor correctly in this case.
The best concept / idea that I came up with is to add an additional tone-ring to the inboard CV joint of a drive axle. Next, fabricate a bracket that would hold a factory GM vehicle speed sensor up to that ring. You would then need to recalibrate your ECU properly match the tone pulses to actual vehicle speed, but I see no reason why this shouldn't work.
Good luck and let us know if you come up with anything different.
Ahnold
Mirada
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