I ripped this off the Swedish Volvo Amazon Club's forum a while ago, since the 123GT rev counter is currently topping my most wanted parts list. Please note that I have not (yet) made this installment myself. I hope this helps!

" There should be one black, two red and two white cables coming out of the Smith rev counter. The black goes to ground (minus on the battery), one of the red goes to the instrument lighting on the lights switch, the other red goes to the 25A fuse (the short fat one) in the fuse box, the one white goes to the ignition coil (instead of the black, which you remove completely) and the remaining white goes to the distributor where the removed black used to be. If this doesn't work try switching the whites. "

Good luck,

:: Fredrik