If it isn't the ignition switch as stated above, I'd suspect the blower motor. The defrost position is the simplest least intuitive position of the system. Fixed duct position, maximum heat, maximum blower, no decisions to make. If it isn't blowing sure sounds like a blower issue.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician