I'm not sure there is a bypass harness for this; even Crutchfield recommends running new wiring. The impedance is odd (6 Ohm I believe), and the amps are wired in a strange configuration (I want to say they're in a series). Other than a head unit, almost nothing works with the system other than OEM parts, and what is available for that isn't worth the asking price.
I rewired my whole system over a blown speaker and would do it again. A simple solution might be to run speaker wires from the dash to each speaker location and put in a decent 6x8 on all 4 corners, using the amp built into the head unit. I did components and a 5 channel amp (and a sub I have yet to build a box for), so it was a little more involved, but still not terrible.