Since the car is going on 20 years old the blower motor is probably worn out. You may find it on the passenger side foot well in the AC plenum/duct (this is the position on the W123's and your AC system is the same type - Type II with some variations). Remove the panel below the dash and you should see the blower assembly. There should be an electrical connection on the blower housing that you can unplug and then run power to see if the blower motor is in fact worn out. Before running separate power to the blower try this, if the blower motor sits on end you should see the armature shaft's end exposed at the bottom of the blower assembly (consists of the blower motor, fan, and elec. connections in a fair-sized plastic housing that removes as an assembly), turn on the blower motor at the control panel, take a hard faced flat ended tool (like a 1/4" socket tool with a handle like a screw driver or a 1/4" extension in a ratchet and push the armature shaft up with the flat end of the tool, if the blower motor spins then you've found the problem (if this works then look up my reply to a post recently began by moderator TXBill about his blower motor problems for how the motor wears). If your brother has a different type of blower motor assembly/mounting position, then my info will only help you get started looking for the problem.
I would suggest not trying to use the blower motor controls to find a way to jump the connections there to try and get the motor to work. Find the blower motor and use that electrical connection to see if it runs - lots safer and lessens the chance of damaging the expensive and complicated AC control panel components.
Hopefully, your brother has the type of blower motor than can be rebuilt because the motor's housing can be taken apart and the brushes replaced.
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