Instead of having music tell us how it works. Note that each pin is numbered. 85 and 86 are the coil pins while 30, 87, and 87a are the switch pins. 87 and 87a are the two contacts to which 30 will connect. If the coil is not activated, 30 will always be connected to 87a. Think of this as the relay in the Normally Closed (OFF) position. 85 goes to the power window swtich which connects to ground. When 85 on the left relay is connected to ground 87 connects to 30 placing positive down the blue wire to the motor. The right relay is at rest so 87a is connected to 30 causing negative (ground) down the purple wire to the motor. When 85 on the right relay is connected to ground 87 connects to 30 placing positive down the purple wire to the motor. The left relay is at rest so 87a is connected to 30 causing negative (ground) down the blue wire to the motor.