First, let's check the head unit out.
disconnect the outputs front and rear from the car's harness.
find a test speaker, a 4 ohm known good speaker.
power on your head unit with good signal or cd playing and connect the test speaker to each of the four outputs one at the time to make sure the head unit is functioning on all channels as it is supposed to be (you should hear sound from the speaker each time).
If everything checks out (if it doesnt, you have a bad head unit barring other problems), let's look in a different direction.

look under the passenger seat in a 2G, some have an amplifier that has a round din-type plug in one side and a molex output plug in the same or other side connecting to a box bolted with 10mm-12mm to a bracket under the seat. If your car doesn't have one of these amplifiers then check the speaker wiring from the head to the speaker.
Otherwise, these amplifiers whether infinity(all molex plugs usually) or OEM, we need to consider the input types. most are differential input but some are common ground input.
What does this have to do with anything?
Some head units will work with the stock harness wiring and work with the OEM amplifier to the OEM/aftermarket speakers (signal sensing amp turn-on). Some will not, if your's will not, you might need to run individual speaker pair wires for the rear speakers to bypass the OEM amp.
Any questions, PM me.
I am not always right, but my intentions are genuine.