The Low Speed

The coil for the relay at the top gets hot from the ignition key

The air thermostat (11) measures temperature of the air behind the radiator, at a certain point, it closes the circuit, energising the coil in the relay at the top.

This relay allows power to flow to one fan (7)

the ground wire for fan (7) goes back to terminal four at relay (13) on the left wheelwell. This relay is a Single Pole Double Throw. Unenergised, it allows terminal three to source its power from terminal four

Terminal three is connected to the second fan (8). The second fan gets "used" power of the first fan. Since the two fans are connected in series, each gets 6 volts of power, and runs at "half-speed"