Don't cut any wires...
Check behind the tacho and there's a connector with 3 wires to it - these are +ive (12v) -ive (Earth) and the pulse for the Tacho.
You only have 4 wires on a RevLight - 12v, Earth, Pulse and the Program wire which you touch to Earth to program it. A tip is to connect this to earth via a push-switch so you can program the RevLight much easier and also change it's modes on the go. This is what I did:
http://horrible.demon.co.uk/fairing/.../040506_02.jpg The push button is above the light cluster.