Difficult to see, more difficult to access.
Same as on Panda. Have a search in Panda as well as 500, there are several threads about this.
You need to stick your head down by the pedals. The resistor is fitted into the heater box, high up, just left of the clutch pedal. Held by a single screw, I think, then it 'just pulls out'. If only it were that simple. Difficult to get a driver on the screw, then once that is out, the resistor may just pull out, or may be quite stubborn. If it has overheated, it may have swelled a bit, becoming a bit larger than its hole. Will need much swearing.
If the fan works only on the highest speed, the resistor is the most likely culprit. If the fan does not work at all, or intermittently, or sometimes they work on all speeds after high speed, the fan motor is more likely. That is easier to get to, being on the left, and more accessible after the glovebox is removed.