believe it or not its due to you removing the air-con... on the piping for the air-con there is an electric block connector that needs to remain connected in order for the fan to work on the normal rad, have a look at the steel piping for this and just re connect the block connectors, you dont need to re-fit the piping or anything
If you leave the plug un bridged or disconnected it leaves the fan running permanently,not the other way around
Id check the wires and connectors on the rad fan.
Is the gauge in the car working ok?