How does the Rallye Dash Tach hookup?

Steve the tach harness is a separate sub-harness, like console wiring, and isn't automatically there.

I do not know if this will help, but I modified your picture to maybe visually help. What Red was saying earlier, if I read it correctly, is to add a fuse connector into the empty slot "add fuse here". Since you have a new harness I assume you still have your old one? You can remove a fuse connector from it. I do not know where to source these connectors yet. Your tach wire would now run to this now added "add fuse here" connector and be fused by itself. It should already receive power from the #3 buss bar. after it is wired all you have to do is add a fuse to the fuse panels empty slot (reverse) and the tach should work - if the rest of it is hooked up correctly.
Manual shows connector should be dead until key is in the acc or run position might want to check that at 3.