1985 GT convertible

Alright well i just bought my car and the tach isnt working right now. It has an msd 6al box installed already and i was told that you can run the tach off that. There is no wire coming out of the whole for the tach so im just curious on what to do. This is my first time dealing with MSD. Is there a wire that i need to buy that goes into that plugin, and can i just get this from the msd site? From what i researched it seems that im also going to need a tach adapter PN 8920. Is this true and if it is, i found a wiring diagram, but should i be looking at the voltage triggered diagram or current triggered diagram? Any help or direction would be great.