As above but make sure that your rev counter will work with your ignition type(especially if changing/defo recommended), or at best it will be innacurate, at worst just not work.
They are all fed from the negative(switched) side of the coil, on fords escort mk2's etc from a extra green wire on the coil itself, that goes all the way back to the rev counter.
What make rev counter are you using ?
tabetha