how to connect electronic revcounter?

i would like to fit an electronic revcounter to the featherlastic i am building but as i am completetely helpless regarding anything electronic i would like your thoughts on this

the revcounter i got ( and would like to use as it fits the dominator as a glove ) has 3 connections
neg 6v
neg 12v
+ (that is what it says on the revcounterbody )

the igition i am going to use is also electronic (to avoid using a std commando timing cover) and has an alternator of wich 2 coils are used as triggers for the ignition
from the alternator/pick-ups 2 wires go to the charging system and , more important , 2 wires go to the advance unit
from the advance unit one wire goes to the ignition coil
one goes to earth ( so far this is wired to be negativ
one wire goes to the regulator , through a relay ,

the rev counter is a magnetic smith , and has 0125/VI painted on its body

any thoughts on this?
should i connect the + (revcounter) to the wire going from advance unit to coil?

You must consult the manufacture's recommendation of which wire the tach can tolerate. Usually its a coil trigger wire or even a HT plug lead. Keep in mind a dual fire ignition can fool the tach into displaying twice the actual rpm. Usually there is a setting for how many cylinders its monitoring to get correct spark to rpm ratio displayed. Might have to isolate the tach if its not positive Earth and grounds through its mounting c

as the revcounter is also over 20years in my hands , i have no info about it as i got it at an autojumble
i do know it could show double the revs as it could be from a car (4cylinder) or half the revs i am actually doing
the fact that it has got a 6volt AND 12 volt connection does say something about the divesity t is intended to be used , i think (or better hope )

Well if it smokes or goes crazy try a different new unit as old one now fried.
Dual fire wasted sparks ignition hits twice as fast as rpms