Rear speakers cut out, cut back in when car heats up from the sun

I have a 2000 Toyota Camry with stock stereo. Recently the rear speakers cut out :crying:. I checked the connections and they are fine. Then I noticed that when my car heats up in the afternoon from the sun, the rear speaker (right) will begin working until I blast the AC and the dash cools. From what I gather this means the connection is bad in the wiring harness or it is something in the stock stereo. Not sure what to do now. Do I cut the wires and re-fit the harnesses or do I need a new head unit? Thanks

never heard of this but keep in mind the stock stereo is 16 years old, most stuff doesn't even last that long