I upgraded my ignition on my '84 Bronco to a MSD 6A box and Blaster2 coil. I can't seem to figure out what wire to hook up so my stock tach works. Any suggestions would be helpful.

 

The "tach" wire always goes to the negative side of any coil(with a negative ground system). The original coil pigtail should've had 2 green wires on the negative side of it,one went back to the Duraspark module & the other went to the dash for the tach. Find the one that goes to the dash and hook it to the negative side of your current coil,or, to the MSD if the MSD instructions say
so.
Phillip