Help! - my blower won’t switch off!!!!

The fan on my e34 (with aircon) has today stopped responding to the rheostat AND also won't stop if set to O, nor slow down if set to the stop just before zero.

Also, if I set it at zero and select footwells, rather than auto, it sounds as if another fan (??) is still running.  The noise of the blower is coming from around the area below the big air vent in the centre of the dash.

Is it a case of throwing away the dash unit and replacing it (Yikes! Sounds dear?) or is there another way.

Only owned the car a few weeks, never had this prob before, all heater options worked perfectly until 2:00pm this afternoon.

Any guidance, help, prayers gratefully rec'd

Thanks