It's certainly not a fuse. There's a very common problem in the Jeep Cherokee around your model year, involving a broken speaker connection in the conduit leading into the door. This almost always affects the driver's door speaker first. The solution is to run a new speaker wire into the door.
The rear speakers don't have this issue, so the problem with your left rear speaker might be related to the speaker itself. It's also possible that the radio is at fault, or that the broken speaker wire is causing a short that's affecting both speakers on that side. However, I'd bet money on your vehicle having a broken speaker wire going into the driver's door, just because I see more Cherokees that have this problem than Cherokees that don't. Eventually your passenger front speaker is likely to be affected as well.