Wiring a car speaker system
This is a typical wiring diagram for a four-speaker system.
The four speaker wires from the radio/cassette unit, which are used for the normal two-speaker arrangement, are first connected to the fader switch input terminals. From the switch output terminals a pair of wires, one positive and one negative, are connected to each speaker.
If a graphic equalizer is fitted to the system you should connect the fader unit between the equalizer and the speakers.