The tach connections are already in the printed circuit on the back of the cluster housing.
You only need to add one wire to make this work. Connect the tach feed (brown wire with inline "filter") from the tach terminal on your distributor to the single white wire connector that you will find in the IP harness. I found the white wire's connector taped tightly to the harness, between the steering column mounting bracket and the top of the brake pedal pivot bracket. It is a single white wire that goes to pin 121 as shown in the wiring diagram above.
Cars that came with a factory tach had a separate hole in the firewall for the brown wire. The hole location is marked by a dimple in every G body firewall; the dimple is on the angled portion of the firewall, about 6 inches left of the center of the firewall and about couple of inches below the upper cowl seam. To feed the brown wire through the firewall, I used a 3/4" hole saw and drilled the grommet hole at the dimple. I think a 5/8 hole might have provided a better fit for the grommet.
You can see the dimple/hole location in the following photos. To make it easier to work in this area, i removed the distributor cap and all the components that were mounted to the firewall in the area adjacent to the distributor.
Here you can see the connector that will mate to the white wire connector in the IP harness.
In this photo, you can see the factory brown wire with the grommet being inserted through the new hole in the firewall.
If you have the OEM "brown" wire with the in-line filter, install the filter by mounting it at the ground strap attaching screw as shown in this photo. You can also see where the grommet and the filter are located relative to the relay connector and distributor cap which have been re-installed in this picture:
It's a bit of a pain to find the white wire connector and to fish the brown wire from the firewall down to the IP harness, but if you take your time it makes for a clean, "factory" installation.
One other thing I STRONGLY recommend...
Most of the factory tachometers become inaccurate over time. In almost every case, it is because the jumper clip (selects 6 or 8 cylinder operation) has lost contact. Mine was bad, too. In practcally every case, soldering in a jumper wire to permanently replace the snap-in jumper bar will fix your inaccurate tach.
Here's a link to the "how-to":
http://tech.oldsgmail.com/eint_tach_fix.php
Good luck!
