Don't know if this is any help, but if it saves you going down the route of the fan controller it may be...

I had problems with my auxiliary water pump a while back. Sometimes it would come on, sometimes not.
When checking for a power feed at the connection to the pump, sure enough when the pump wasn't running there was no power being fed there.
BUT... it turned out to be the pump that was at fault.
It seems the fan control unit would only supply the power when the load of the pump was there, and as the pump was playing about it wouldn't start the pump every time.
Almost seemed like you need to match the resistance of a fully functioning pump at the time it should be activated in order to get the power supply to test it, but I could never prove this so just took the plunge and took a chance with changing the pump, and sure enough it's faultlessly run every time since...
 
Interesting that! Only ever heard my pump once in 6 months but powers up and runs fine from a direct feed. If the part number I quoted b4 it right then looks like can get one for £20 off ebay so not to damaging if not the fault.
 
not yet! put my car on the ramp at work yesterday wanted to double check the part number couldn't see one stamped on the module mine is defo under the battery tray on passangers side. So all I got out of it was finding out my n/s outer cv gaiter is leaking! Great

Let me know if it fixes it.