There are 3 ways to obtain a engine speed reference signal on a OBD-II LT1. You can either tap into the green (negative) coil wire, or the white 20 gauge wire (pin 13) located within the red pigtail connector coming off the PCM. These will both give you accurate "engine speed reference signal". On OBD-II's you can also tap into the white w/ black stripe wire off the ICM.
Since the late model GM OBD-II cars use a two pulse per crankshaft revolution, the white wire off the PCM is viewed as being the most accurate, but all 3 above will give you an accurate ESRS.
As far as the hot lead, any switched power source is fine.
If you want your tach light to dim with your dimmer switch, you will need to run the tach backlight wire to the dimmer switch or the dimmer hot lead that runs to the stereo.
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