The factory tach harness will hook up as follows.
Your signal wire comes from the + side of the coil. Disconnect the + at the coil, run a wire from here to one of the while wires on the back of the tach, run the other white wire back to the + on the coil. The factory harness also took power (green I believe) from this same wire coming from the coil. It's a positive source originating from the ignition switch.(I know it sounds crazy, you just did a loop around the firewall, but that’s how Nissan did the tach harness). And of course there's a black wire that just needs to find a ground (see further ahead for instruction here). The tach has an inductive pick up, like a timing light, and there is no direct connection to the tack (no loss of power except for the minor voltage drop over the length of added wire).
The last two wires to hook up will be the lights. Under a blue piece of tape in the main harness in the area of the tack's mounted position (look way up to find the harness in metal clips right below the dash pad), you'll find both a light wire and a ground wire that you can use for this purpose. I have always managed to scavenge a plug from my pile of old wiring harnesses to match the one found in the stock harness (I hate cutting anything to do with a wiring harness). This makes the installation a little more professional looking, and more reliable in the long run. But I have to admit, getting those pins out of the stock plugs is a lot of work.
Good luck, it's all there, write back if you have issues.
As a side note, the two white wires that you hooked up in series to the coil are both identical, but the direction of current has to be the right direction, so if you made all the connections, and the tach doesn’t work, just swap the wires either behind the tach, or at the coil, the inductive pick up gives the right signal in only one direction.
Byron