There's probably not really a whole lot of written material about this. Only thing I can think of would be to get the factory Subaru service manual, or a subscription at www.alldatadiy.com and you can get the factory wiring diagram.
But if you can use a multimeter or test light, it shouldn't be that hard to check.
Also, look on the Relays section of this site, and you should be able to find a diagram to use a car alarm, two relays, and actuator(s) to add power locks to a car that didn't have any before---the factory system in your car should be similar, just substitute the switch on the door instead of the car alarm.
Anyway.....
The system is fairly simple. You should have the following in the car, in this order:
1: Something to trigger the door locks. Usually the switch on the door, a remote control, or both.
2: Something to take those triggers and distribute power to the actuators. Usually it's two or three relays, but they could be built into the car's body computer.
3: Wiring coming out of this control system, going into all the car doors.
4: An actuator in each door, with two wires going to it.
So........
You know it can't be #1 or #2, because your other door locks work, right? But it could be #3 or #4. Either the wiring going into the door has broken, or the motor is bad.
It's probably the motor, but the goal is to test and verify before working, not just throwing parts at it.
Do the following:
---Take door panel off.
---Find factory doorlock actuator; it should have two wires going to it.
---Test that both wires rest at ground.
---Test that one of the wires turns to positive when you lock the doors.
---Test that the other wire goes to positive when you unlock the doors.
---Alternately, connect a light bulb or test light to the two wires going to the actuator, and see if the bulb illuminates when you operate the locks.
---If the wiring doesn't check out, follow it back into the car somehow. There's a good chance they've broken inside the door jamb boot from all the flexing.
---If the wiring does check out, then the actuator must be bad.
Aftermarket actuators are very inexpensive, but they sort of have an Erector-Set quality to them. You would just leave the factory stuff in place, find a good new spot to install the aftermarket, clamp it onto the factory lock mechanism, and connect it to the factory wires.
The aftermarket actuator is easy on paper, but needs a good amount of patience in real life, to find a spot where it will work well, but won't interfere with the window or door panel.
The OEM Subaru actuator should work well and be a little more straightforward to install, but it will be more expensive.