Do you really want to keep this radio??? its only worth 129.99 brand new!
If you do here's what to check first:
Make sure the radio
Fader is set to front and rear first!! Alot of the time these radios are set to front only.
Have you made sure that the rear speakers work? Have you tested them?
Have you hooked up a known working speaker to the speaker leads going to the rear speakers to make sure the wires going to the rear speakers work?
If this does not work let me know and I will tell you how to take the radio out. Make sure you have a hack sawblade...

My son liked it so ignore any meaning!!!!