At the highest fan speed, the current does not go through the resistors,
so they would not be the problem. Most likely the switch is at fault.
This can be tested easily, there should be 5 wires going to the switch:

Brown/White (supply)
White ("1")
Green/Yellow ("2")
Black/Green ("3")
White/Red ("4")

Your wire colors may be different, VW used different colors for
different years/production site/ model (Golf/Jetta)

Unplug the wire harness from the switch and short together the
Brown/White and the White/Red wires. The fan should come on full blast.
If it does, the switch is your problem. There is also a junction where
the White/Red wire from the switch and the White/Red wire from the fan
motor meets the White/Red wire from the resistor pack (don't know where
it is located, follow the wires from the fan motor). If this junction
has loosened up over the years, you could loose either a) the highest
fan speed, b) the three lower fan speeds or c) all fan speeds.

The refrigerant low pressure switch locks out the compressor clutch, but
does not affect any of the fan speeds.