need help installing monster tach

i bought a used autometer monster tach 3904 with no instructions! i hooked everything up and have power but the rpm's wont work on the tach.  please give me info on what to do to make the tach read the rpm's!  thanks   its a 90 gt  

If I remember correctlly red wire is 12 volt power,
Black wire is ground, Green wire is to negative side
of the coil. I believe you can also go to autometer
web site and down load instructions. I'm thinking that
if you run the green wire direct to the negative side
of the coil it should work even on that ignition. Or
do you still have the stock ignition, if not what is on it???  

i got it working this morning thanks for the help