Hi Peter,
If I remember correctly I think that these are controlled by PWM. I have had one of these that had corrosion within the signal wire insulation near to the multi plug. I replaced the offending section of wire but a year later it happened again, this time at the point where it went into the controller, so the only answer that time was a complete new controller/fan motor assy which I think was about £160. Water must have got into the wire at some point and worked its way along. Try scoping before and after the plug and near to the controller, or a multi meter with duty cycle capability should do just as well.
HTH's
Bryan
PS. Have just re-read your post when a bit more awake. I'm sure the controller wire is PWM switched earth which would explain the lack current when disconnected, assuming you are pulling the multi plug apart. If I am correct and you have batt volts when all connected that would mean a lack of earth, either in the loom or from the PCM. The signal should vary from 0v (fan running full speed) to 12v (fan off) but as I have said it's best to scope it.