If you pull the cluster out you can tie everything into the right side harness, except the ground. I just ran it to the screw on the fuse box under the dash. In order to find the wire on the right side harness just turn your lights on with the cluster out, ground the gauge by holding the ground wire against the door frame or that metal tube behind the cluster, and then touch the 12v lighting wire of the gauge to the various contacts, one at a time, on the right side harness. When the gauge lights up you have the right one.
For the 12V ign. wire just look on the right side of the cluster. Near the middle you will see "IGN". The "IGN" will be encircled by a gold or copper wire. Follow the wire down to where the harness connects and then match up the gold colored connection with the corresponding wire of the right side harness. You can also just turn your key on and the touch the 12V ignition wire of the Autometer gauge to the open contacts on the right side harness with the gauge still grounded first. When you see the needle of the gauge move up you know you have the right IGN source. This sounds really crude, but it is how I discovered the way to install mine and also some gauges on my friend's Z28. It also allows you to hook up all your wires to one spot, which makes problems more easy to find.