I had a nightmare with doing this! I tried all sorts, the alternator has a connection for it.. that was way out, coil feed wouldnt work, signal cable to stock counter might work but couldnt find it... ended up taking it back, but soon after some one figured it out and did it so there must be a way!
I know thats not helpfull but just so you know... the usual places to connect dont work

Trace down the ECU signal cable.