Internal balance adjustment?

I have an old Pioneer KE-2323 cassette stereo in my car and the left is louder than the right. balance adjust helps, but it's definitely something with the radio itself. Is there a 'master' balance control or something somewhere, maybe inside I can adjust? I'm handy with soldering etc, so if anyone can point me in the right direction, I'd appreciate it!! It's an old Jeep CJ-7 and I want to keep the original shaft-mount setup.