I guessed they were using MOSFETS, which frequently die when used in car fan speed controls.

Hopefully there is a heatsink on the thing. You can help it last longer by mounting it where air will blow across it.

FETs sometimes fail by shorting (fan will go all the time.)

If it does die again you can replace with a higher rating if you can get the heat away. The cost difference is about ten cents, nothing from your point of view, but that's quite a bit for a truckload of devices.

97 T100 4x4
97 328i
95 540i restoration project