SOMETHING has damaged/or (still) overheats the L output.....if you are lucky the output is protected and is tripping from overload and it cools down when you leave it be, and trips or fails again......the most likely culprit is the orig LR speaker......absent that, look for short to ground along the spkr wires on the L side.....if so, it won’t be a dead short, there will be some resistance. I think probably this will end with the realization that the L output is hurt, though, unless it has protection (thermal switch that opens) as some high-end equipment has.....
What confuses me is that (with the bad speaker gone) it’s now the left-rear only, and not the entire left side.
I’ve never heard of that.