MK3 Fiesta Clocks

Hi

I've put a set of clocks from an N reg mk3 fiesta ghia with HCS engine into my wifes G reg mk3 fiesta L so that she has a rev counter. Every thing works ok, and the speedo calibration is fine except that the rev counter doesn't work. I'm sure people have done this conversion does anyone know if I need to add a sensor and wire from the donor car and if so which pin of the multiplug does it go onto.

Thanks in advance

Dave