It may be a good idea to check the wiring at the power amp end. Its possible that an aftermarket stereo was fitted by the previous owner of the car and then the standard one put back in before it was sold on to you. In doing this the wiring may have been messed around with.
The amp is behind the dash on the lefthand side. open the glovebox and squeeze the sides in carefully, this will allow it to tip down completly. the amp is up in the top lefthand corner. It will have a single white multiplug in the bottom. Check around this for wires that may have been cut and joined.
Also try a simple circuit tester across the terminals on the rear speakers if there is no continuity between the two terminals the voice coil in the speaker may be burnt out.
Feel free to ask me any questions on this as I used to fit car audio. The is200 setup is unusual and many car audio places may not have dealt with one before!