Hey fellas.
Spent a few hours on this over the weekend with the multimeter. Completely did my head in!
The 2 wires (+, earth) coming out of the fan EACH had connection to battery +'ve AND earth with engine/AC on. E.g. the multimeter beeped on both battery poles on either wire. The fan didn't work (obviously...?!).
Not sure what's going on apart from a short somewhere. Will post again once I've sketched out my own diagram, or given up and hard wired the thing to run with the compressor!
Cheers.