The tach signal wire is typically attached to the coil - terminal, but there are some applications where this is different.  I believe the factory rally pac wiring ran ignition power to the tach and then from the tach to the + side of the coil through the pink resistor wire.  If its a factory style rally pac it should be wired like that, but if its aftermarket I can't say.  Just be sure its hooked up correctly either way because getting it backwards is not going to be good for anything involved. 

Did you bypass the pink resistor wire as we advised?  If so that's your issue.  The trick here is I don't know if its okay to connect the tach to a wire that isn't the resistor wire.  It may need that resistance.  If it does its not a big deal, just put a resistor inline between the tach and coil that's the same resistance value as the pink wire was (you can measure that with an ohm meter). 

On second thought....did you replace the stock distributor and coil with some sort of "better" one?  If so, and if you have the stock OEM tach, it's possible that the tach will not work with an aftermarket distributor/coil.