The fan is easy to access under the black plastic cover, center rear of engine compartment.
I don't remember where the resister pack is by if you can get to the two wires that power the motor verify that they have battery voltage across them.
In other words the motor runs in high speed with battery voltage across the two terminals of the motor. Reduced speeds are achieved by adding resisters such that the motor only gets 6v or 8v or 10v say. measure the voltage, or power it up with a jumper to 12v (only do this with the motor mounted as it will take off in high speed.)
If the motor gets 12v (battery voltage) and is slow then it is bad. If the motor never gets more than 6 or 8v then the control of blower speeds is faulty and that diagnosis will require a wiring diagram.
Bosch Master, ASE Master, L1
33 years MB technician