Sorry, this is Longhorn Hoss (just couldn't find my password so I had to use my other login name).
To answer your questions, my truck is a 97 XL F150, 4.2 V6 4x4. I did not have the tach in the truck when I got it, really strange because it is a standard (I've been made fun of before for it).
I found a post from around June of 99 in the aftermarket mods section that led me to remove the instrument panel. Also, many of my friends have told me that the way most car companies work, they usually use a generic wiring harness for all vehicles, then just place the correct instrument panel based on customer needs. Mine had the tach wire in the back, along with a bunch of other wires I didn't care to research into what they were.
Oh, set the switch in the back to 6 (mine has 4,6,and 8). I will say that I had heard that the correct wire, and this was from another post, was the white with pink striping. Just to make sure, I hooked up the power to the tach, then took the signal wire and tapped it to the back of the wiring harness. Revs showed up perfectly.
One last thing! Make sure that you connect the BLACK ground wire to a ground and the GREEN wire to the tach. I was in a hurry, and mistook the GREEN for GROUND! I had the engine running and while I was tapping the different connectors in the back of the harness, one happened to spark, and kill the engine. DOH! Nothing else happened, she started right up, and ran fine, just a little scare.
By the way, what year is your truck?