Its the only complicated bit of wiring on a series.
If it's only a one speed motor you only have two lives to worry about.
The first live from the switch, activates the motor. The second permanent live takes over when you switch off and continues to power the motor until the park switch on the motor is activated. The park switch is inside the white plastic box on the motor that the spade connectors are in.
You can burn the park switch out if wired wrongly so check the wiring first via a wiring scematic and if no luck buy a new park switch. They are about £15.