I think ones power, two go to the tach, and one goes to the - terminal on the distributor plug. (Could be and most likely wrong, it was like 4 years ago.)
I installed a tach on my 90' XJ a while back, and it really was that simple.
I know on the split second psc1, I had to get the tach signal from the cam position sensor output(I tried tapping the tach behind the cluster, but was getting an inaccurate reading: 250-300rpms less than actual).
