The fan under the dashboard that supplies air to the vents has 4 positions.
Position 1 and 2 sends the full power through a resistor, this lowers the power
to the fan so it spins slower. The fault is with the contruction of the resistor pack
so you have 2 possibilties, change it for a new one or fix the old one.
My Heater/Cooling Interior Fan Doesn't Work On Any Settings
Check your fuses, if they are fine it will either be a dodgy connection to the
blower motor or the blower itself may have burnt out.
Resistor Pack Removal
Smart seem to want to charge big money for this fix so it's well worth doing yourself.
On a RHD car, look in the passenger footwell, to the back and to the centre.
The LHD smart has it in the same position but in the drivers footwell.