a tach can be fitted to 95 and earrlyer mgm through the ignition control module. does a 96-up have a ignition control module

 

I'm pretty sure they have a powertrain control module that controls timings for everything. I'm not sure though. I can't find a pinout for that right now either. There's probably a tach lead on it somewhere. If yours has such a beast, it should be on the driver side fender wall of the engine bay near the cab area I think.