Personnaly I would never just increase the fuse rating, although in some cases the wiring will stand it but the original fuse was badly chosen, it has happened before - maybe with Fabia rear wipers(?). Normally with a heater motor, the space between the motor segments, which should be open and insulated from the next segment, gets filled with carbon from the brushes - and so conducts. This can cause the motor to fail to rotate at switch on and blow the fuse quickly. If it is possible to open the motor up, then you could clean it up, in particular check the space between the segments. Of course it might be some of the motor's internal wiring that is causing the short - or maybe its not possible to re-build these motors after opening them up. That would just leave you needing to buy a new heater motor.
Edit, worn or "dry" motor shaft bearings can also cause a motor to draw too much current - had it been a bit noisier than normal prior to blowing fuses?
Edit again!, if the ballast resistor assembly is dying then the motor would not run at slower speeds, normally the slowest speed fails first as the most voltage is dropped across the ballast under that condition.