digatron power/tach connection M1000

I am hooking up my last years digatron to my new M1000 and forgot which wire in the harness it is suppose to be connected to. There was a thread on the old forum that explained this issue last year. I remember that I need to find the wire for the tach signal so that the unit will sense rpm's even if they are wrong. Otherwise the unit will shut off to save the batteries if it does not have that tach signal. Yea, I know but this is the model that I have!.. Any help with this is appreciated.