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The best values will depend on the signal levels for your car. The reason you see different values published is there has been some experimentation trying to find values that would work with most cars.
You will not cause damage with either value. I would use the 50K. If you have signal problems try adding a resistor in parallel with the zener diode. Try values between 10K and 50K for the parallel resistor.
On the input circuit the input resistor and zener diode combine to limit the voltage on the input pin of the processor. Sometimes the signal does not swing low enough to register as low on the input pin. Adding a resistor in parallel with the zener helps pull the signal low enough to register as low.
Mike
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