After pulling out the cluster to remove the clock and replace it with the tach, look in the nest of wires back there for a loose one with a female spade connector that's in the right place to be hooked to the male spade near the bottom of the tach. That wire seems to have different colors depending on year, but on our 83 it was a Red/White...nicely camouflaged inside a Gray sleeve. The tach's power and ground connections are made automatically when you fasten it to the cluster.

The trio of male spade lugs at the top of the tach's rear are meant to connect to a short wire harness and carry constant 12V, ground, and lighting current to the small clock that accompanied the large tach on cars so equipped.
--
Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD; plus grocery-getter Dodge minivan, MGB, and numerous old motorcycles)