As I understand, Defrost is supposed to engage full blower, full heat, no exceptions. If that doesn't get the blower motor running, it may be suspect.

Perhaps a fuse, perhaps the speed control resistor, perhaps the blower motor brushes.

Fuse sounds easy, resistor and brushes require some disassembly for further research. I have done nothing to mine, so I have no experience.

Best of Luck
Jim