2013 JBL system...left speakers cut out...head unit or amp (or something else)?

I'd appreciate any advice on this issue that has bothered me for years on my 2013 5th Gen with the factory JBL system. I've had this vehicle since 2014. At about 114,000 miles currently. No modifications, no serious accidents. I live in Upper Michigan without any top shelf audio shops or terrific Toyota experts (that I know of).

My driver's side audio will cut out almost entirely, seemingly without cause. It seemed this started with the dash tweeter, but it seems to be the tweeter/door speaker and rear door speaker more recently. At times it feels like all audio is out on the left side.

When it happens, if I fade the unit all the way to the left, there is still the faintest audio when I turn the volume to the max (so it is not a complete disconnection/failure). I put it back to the center and eventually it balances out (or close to it), but eventually it cuts out again. Only on the driver's side. There is no pop or interference. It just happens. At times it is a fairly quick in and out, other times it seems balanced for days, then drops on the driver's side again for days. Happens in cold or hot, seemingly not related to bumps or starting/stopping. I does not matter if I am listening to the radio, bluetooth, or a CD.

Things I have tried (open to trying them again):

• Full reset on the radio
• Removing the bluetooth/paired phones
• Pairing different phones
• Turning party mode on/off
• Turning the DSP setting on and off
• Adjusting the balance/fade
• Disconnecting the battery
• Taking it back to the dealership and having them try to diagnose it (waste of money).

I am fairly convinced (but open to suggestions) that this is an issue with the head unit or amp. Any opinions on which one to dig into? I have third row seats so I know accessing the amp is not an easy task, but I could get to it if I had a free afternoon. I have not pulled the head unit to check any connections, and I feel dumb typing this and admitting that.

Open to any input, and curious if anyone has had the same issue. I'm tired of being frustrated with what has otherwise been a great vehicle.

I appreciate any help. Cheers.

Josh