Took the rear panels apart this weekend to get the thing tinted, and since I was there I noticed there are 4 rear speakers. 2 the PO had replaced with some 5.25" Sony's and the other 2 in the shoulder area are stock. All 4 are working. Now when I installed the Alpine head unit I hooked up the speaker harness like the diagram said to. but it's only a 4ch head unit, so how am i driving 6 speakers? Bit confused here. Is there a "cross over" in the rear wiring?