Need help with tach wiring

I recommend splicing into the wires and soldering and then use shrink tubing over the splice. If there is a location to use a plug type end on the wire, use that for the connections. The connection for the tack on the MSD may have a plug type or a bare wire to splice. For a plug, buy the type connector you need. Do not use electrical tape because some day, at the least likely time, it will come loose and cause a possible short and, POOF, there goes the wiring up in smoke. Do it right the first time so you do not have to wonder when it will go bad.

Instead of splicing you can simply build a "Y" adapter and use bullet connectors

Get your self some large nose bullet connectors. If you can't find any you can open the nose (with an awl stuck in the end) of the standard size to fit the stock Ford harness.
Make a "Y" using one male bullet as the base, two 2" pieces of 14 gauge wire, and two female bullets at the top.
To connect all your wiring:
you will disconnect the bullet connect that connects the red wire to the pink wire under the dash behind the key switch. Plug the male bullet of your "Y" connector into the red wire to the key switch. Plug the pink wire into one of the open arms on the "Y" connector you made. add a male bullet connector to the Red tach wire and plug it into the other open arm of the "Y" connector. Somewhere right in the same area as that you will find a light blue wire with a red stripe that has a black rubber head with three open female bullets. This is your dash light wire accessory tap. The White wire plugs in here. The black wire gets an eye and can be grounded to any of the dash screws (I put mine on the mounting screw for the interior light between the light and the dash).
The wire that goes to your windshield washer comes through the firewall in a molded grommet. If you unplug the wire from the washer pump, and take it out through all the ties, you can push the gromet to the inside of the car, don't pull the wire all the way through the firewall though just far enough in to the passenger compartment that you can work with the grommet. Now poke an additional hole in the gromet to feed the green wire through about two inches. pull the washer pump wire back into the engine compartment just before seating the grommet, pull the green wire through into the engine compartment. Now seat the grommet back into the firewall. Run the washer pump wire, and the green wire, back through the ties. Plug the washer pump wire back in, then route your green wire over to the negative coil post. Leave an inch or so of slack for engine movement. Add an eye to the green wire and secure it to the post.

Start the car and check the tach against a known good tachometer to make sure you've got it set correctly to 8 cylinders and that it functions correctly.

Pat yourself on the back and drink a couple of beers while you marvel at just how good you really are!

Good luck,

Scott

If you could post some pictures that would be super helpful

I've answered this question for a few people, and have never had pictures to go with it.
This would be a great oportunity to tie the whole thing together so that people using the search function would have a great thread with a couple of dfferent options on how to wire up the tach. A proper soldering, and adding heat shrink is the best way to go. Using the bullet connectors is totally legit, and easier for those without good soldering experience.
Using solder type bullet connectors, and eyelets, is better than crimp connectors and is on par with a factory add in harness.

Sorry I missed the part about the MSD box. Thankfully, you are sharp and caught that.

Look forward to seeing you job completed!

Scott

I though I would beat you to it Runt! Here is the thread I made and Runt told me exactly what to do. Installing my Sun Super tach USE THE BULLET CONNECTORS! Thats if you arent that great at soldering, like me. Scott H. is the same person as 66Runt. Hope that helps!