Listen carefully to hear if the lock actuator in the one door is making any sound. If it's making sounds it's "trying" to lock and unlock but perhaps the mechanism needs lubrication or something has come loose. If there is sound from all the doors except that one, where it's totally silent then it's probably electrical - could be the solenoid or a connector.
When installing aftermarket speakers or window tint, an installer may have not correctly re-attached the lock mechanism or it was attached in a way that it can come off.
In the Gen 3, generally, the rear door lock actuator motor on one door is on the same circuit as the opposite rear door and the front passenger door. If the others work, it's not a fuse issue.
There are threads here where the actuator mechanism (small electric motor and a set of gears) on the door latch has become seized by hardened grease, or the motor brushes are bad. In either case, there might not be any noticeable sound from the actuator when a lock or unlock command is given.
Standard sort of approach might be to disconnect the door wiring harness at the two-wire actuator connector, and measure the voltage between the two wires on the harness side, when a lock or unlock is selected. There should be a 2 second pulse of battery voltage. A good DMM should be able to follow this, but lesser ones might only show a change from zero to a few Volts at best. This requires removing the inner door trim panel. It could be difficult to reach the actuator connector on the door latch, which is at the far rear.
YouTube probably has videos on accessing the latch mechanism.