"They all do that Sir." French made motors.
It is usually caused by the motor shaft bearings drying out and seizing up. The motor is harder to spin, draws much more current through the resistances, resistances get hot, blows the thermal fuse. If the thermal cut out is bypassed, then presumably eventually the dashboard catches fire!
But if you are reasonably good at electrickery or know a mate who is, buy some generic thermal fuses at some domestic & catering appliance repairers, about 100c centigrade would do 60 pence each?, solder one of these into the resistor pack, use meaty pliers on the fuse wires to keep the heat away from the fuse itself. Oil the bearing with some heavy (non smelly!) oil, the bottom one is easy. Top one is behind the fan, but I just stand the thing on the fan, fill the "dish" behind the fan up with oil, and let stand for an hour. Once done, tip oil, and spin off the excees. Dont get oil on the windings or brushes, else it will die. Repeat in three years time when the bearing dries out again.