When you say unplug the front channels, do you mean unplug the rca's or disconnecting the speaker wires from the amp? Usually when you wire a 4 channel amp, you grab the front speaker lines from behind the deck and run those two lines to where the amp is located. For the rears, you just wire them directly to the amp. (So you dont have to waste 30ft of speaker line running the rear speaker leads...seeing that they're already back there) If you're removing the front channel speaker wires and the fronts are on but the rears go out....just switch them around.
Also, have your front speakers been replaced? If you look at them are they coaxials? If they are coaxials that would explain your tweeters being disconnected. If you connect coaxials with your factory tweeters you are dropping the impedance of each side and the amp may not be able to support it.
