pretty sure the ecu has a tacho output, then to run through the standard dash, will have to change a resistor on the back of the cluster - did it years ago so can't remember the exact details of the resistor swap. the tacho signal wire to the dash originally comes from the coil and was a black wire on my ke70, more than likely same for ae86 - just connect that wire stright to tacho output on the ecu...
found with a quick search this link - although i remember a step through with photos - could probably find it with more googling!
http://forums.club4ag.com/zerothread?id=68162