Th eproblem with the fan being permanently on is two fold - first the engine will only ever reach the min temp at which the thermostat opens, & take longer to warm up, secondly the fans will consume power which you'll pay for in mpg
My 164 had similar symptoms - once the engine got hot enough to switch the fan on, they stayed on permanently. Sometimes even when the engine was started from cold afterwards
The cause for me is the radiator fan temp sensor.
A check would be to disconnect the sensor when the fan is misbehaving - if it goes off, its the sensor.
On the 164 the connector is easy to see on the top back edge of the rad - dunno on the 166
Easy enough to change on the 164 - it's just screwed in to the rad - again dunno on the 166.
My present bodge is a manual switch, run into the cabin. changed the sensor about 6 months ago, so I'm suspecting something has caused the sensor to fail. Works well enough, if you remember to watch the temp.
Does anyone know whether these sensors are vulnerable to voltage/current/over temp/vibration??