06 GT Rear speaker problem

Hello everyone. I just had a kenwood dx770 head unit installed in my gt with base audio and the rear speakers dont work. I previously had a pioneer head unit and everything functioned normally and when the new one was put in the rear speakers just stopped. I verifed they wernt working by ajusting the fader. When i shifted the fader all the way to the rear the front ones got quieter but the rear ones didntnt gain any sound, and when i shifted it to all the sound in the rear no sound came out at all. I had the head unit professionally installed and im wondering if the installer messed up or if im not doing something right with the head unit. Please help