I take it you've attached the sender unit, and need to know which wire in the big circle plug (or the little one) on the instrument cluster board to plug it into, right? That's easy just follow the circuit tracks from the gauge. One would be earth/ground, and the other would go straight to the sender unit i think? I could have it completely wrong, but my cars a whole 10 metres away from me and I gotta go to work now!
AFAIK, there are only two connections to the gauge on the cluster, and you wouldn't need the wires from the external gauge anymore. Could be wrong but...