Where do you hook up the tach wire on a 1999 b2500 pickupgreen wire?

If you are talking about the stock Tachometer, the connectors are in the Large white connector block at the top rear of the instrument panel. If you are installing an after market tachometer, you should be able to tie into the same wiring harness.

Note: The dash is a Brick wall to get through. Don't try to Houdini your fingers up behind the dash. They may disappear forever, or you may have to call 911 to get unstuck. There ain't no way to get back there. Save yourself time and frustration. Take it apart.