The left and right side speakers share a common ground - one ground per side. When you move to a higher output deck you need to separate these grounds as the higher-wattage decks drive the channels separately. The wires, luckily, are tied together near the center console - remove it and you can find the speaker wires, cut the common ground, add new dedicated grounds, and off you go. This way you get dedicated LF, RF, LR, and RR channels.

Been there, done that -

Brian