I found that the live connection on the back of the fan control knob had melted and no fan speeds were working. It's a bit of a fiddle getting to the back of the knob. Pull off the wiring connector which will be difficult if it has melted together. Cut off the burnt out bit of the connector and reconnect the live cable with a new spade connector (if possible), then push the rest of the existing connector block back on.
If it is not the control knob then the way the heater is wired should allow the high speed fan to work even if the speed control resistors have gone. The No1 and/or 2 speeds will not work with the resistors gone.
If connections are OK then maybe it is the fan motor. This can be tested by removing the plastic trim cover over fan in front of windscreen (drivers side) and pulling out the wiring connector from the fan together with the resistor unit which the connector plugs into. Put 12v onto the red and black connections and the fan will work if it is OK.
Hope this is of some use.