Digital display so you have an electronic motor speed control module which sits in the fan caseing and is cooled by the airflow.
The module can be water damaged, what happens is the drain hole for the air intake plennum gets blocked with leaves and rain water builds up until it gets to the level of the fan intake then it trickles into the control module.
If you remove the glove box you will see the module, it is about 2" / 50mm square and clips into the fan housing. Remove it and prise off a small plastic cover by the terminals. If there is furry corrosion inside then thats your problem.
To confirm it, follow the wires into the fan motor, find the two thick ones and apply 12V to it, you should get full speed fan if the motor is OK.
Modules cannot be repaired, you will have to find one from a breakers.