Those are speaker outputs not inputs. Get some electronic contact cleaner, with the amp unplugged, pop the bottom plate off and spray some in the speaker selector switch. If you don't want to do that, you might try rotating the switch all the way to the right and back to the left a couple dozen times with the amp off. You might try that with the volume & balance also. These controls get dirty over time and can create these problems.
Right Speaker input not working!
I recently bought a Soundscraftsmen 3000A Solid State Stereo Receiver along with a Kenwood KD-3100 turntable. The issue I am having is with the speaker inputs. It has inputs for 4 speakers. The two on top are for left speakers and the two on the bottom are for the right speakers. The left speakers work perfectly fine. The bottom inputs for the right speakers are the issue. They will work for about 5 minutes and then it will sound like the speakers are blown out. Everything is wired correctly. When I wire both speakers to the left they work perfectly fine all day. It's the right speakers that are the issue. Check the fuses too and they're brand new. Would anybody know what might be wrong?