hello mate,

do you still have the clocks and pugs from the xr3? for the purpose of tracing the wires put the xr3 clocks back together so you can follow the circuit board on the back of the clocks

if you trace the wires from the rev gauge back through the clocks and onto the the wires on the xr3 plugs. The wires on the plug will be easier to find what does what as, feed to the coil is normally (if not always) green earths are normally (if not always) brown lives can be any colour but reds favorite.

If you look at the circuit board on the back sometimes earths or lives are "common" there will be more than one thing wired off the circuit power from the same circuit earth from the same circuit etc.

A wiring diagram would help for the exact colours but hopefully what i told you helps!

good luck