Wire diagram. Alpine 7164 cassette/radio. Has Bal. and Fader control . but appears to have only 4

Question

Alpine 7164 cassette/radio. Has Bal. and Fader control . but appears to have only 4 speaker wires [ Grn-wht, grn-yel,gry-wht, gry-yel ] grn-wht & gry-wht share terminal @ radio and act as gnd for 2 speaker sys. Only other wires from radio are red- 12v off-on, yel- 12v memory, tan - amp trigger, blk to chassis gnd. How is a 4 spkr system wired to this unit ??