This issue has been going on for a while. I installed 4 new speakers and it still didn't fix it. The rear left and front right speakers do not work. I tried different speakers, and I checked the connectors on the speakers and the head unit and they look fine. Could it be the head unit itself that's not working? I'm planning to install a modern-ish single din radio in the future, but I want to get this problem figured out first. Any suggestions would be great!

Thanks,
Tom