The easiest way to do it is as i did it.

I have a MK3 cabby, which was an XR3i, i patched in S2 wiring into the original loom and removed all the redundant wiring:



Thats the finished article, i left the coil wires outside of the loom like a proper RS loom. I even wired in the KE Protection module into the fuse box (yes it was very fiddly though ) A bit of time and care and no-one would ever know