Explain how a permanent magnet windscreen wiper motor achieves 2 speeds?

Almost certainly due to the Voltage allowed to the motor by the controller (resistor).

Adding a resistor will work, but is inefficient as the unused energy is wasted as heat.

Two speeds are achieved by using a third brush,

With the standard brushes in use. 180 derees around the commutator, current passes through all the rotor windings. The resistance sets a specific current draw and the motor runs at a standard speed.

The high speed switch position feeds power in tothe motor through a third brush set at about 160 degrees from the earth brush. Current now passes through fewer rotor windings - less resistance - more current flow - higher motor speed.