Ryan,
I got my AM ProComp Tach to work.. but I was using a different pickup.. I have not gotten it to work with the stock pickup.. (I only had a few mins to try it with the stock sensor.) It worked on 5 or 12V, but I had to change resistors, when changing voltages. (It was a 3 wire pickup.)
If someone can tell me which wires are what on the stock pickup, it will save some time.. (I don't have a truck to measure the voltages, I'm just running it on the bench for now.) I think they were
black with Blue stripe
gray
Purple with ??? strip
I thought it was
Ground
signal
Power
but didn't have any luck.. but I didn't have time to try different resistors..
Bryan