If you have the automatic digital readout temperature control your model has a blower control module which replaces resistor packs on the manual model. Those can fail in different ways where they sometimes work, sometimes don't, change speeds erratically, and stay on is a failure mode.
Follow the leads from the blower motor up to the relay/maxifuse panel to find the BCM. The blower module is bolted into the channel carrying the air from the blower motor because the power transistors have to be kept cool. Depending on the layout, you might have to loose bolts holding the maxifuse panel in place to lift it slightly.