I have had the issue with the rear doors failing to lock/unlock, I have the keyless entry type of door locks.
First noticed on the nearside rear when I gave a friend a lift and, the door had to be unlocked manually, as the little lever did not active from the central locking system.
Shortly after that I started to notice the offside rear door not locking and unlocking intermittently, and after a while could only be locked from the inside pushing the little lever in.
I found online some replacement actuators after doing some checking to find out what the problem could be.
Here is a link https://www.ebay.co.uk/itm/232792541799 ,I ordered a left and a right side total charge £124.16 with postage.
I fitted them myself, the first one was a little tricky trying to work out what to do, the keyless entry electric connection stumped me for about 10 mins till I sussed out the workings of it, there is not a lot of room to view things and my failing close up eyesight is a pain in the rear end.
The second one was a doddle now I knew how it all went together, I did not have to remove the windows, though if I remember correctly I undid the bolts that secured the window motor to the mount and this gave enough room to get the door lock assembly out. It took probably an hour each side not rushing at all.
The new ones have been in and working with no issues since October 2019.
I took the old ones apart to be nosey and saw there are 2 small electric motors that operate the mechanisms. I had a look on Ali express and found replacement motors the code numbers are FC280 and FC130 there is 1 of each in the actuator, I bought 2 of each for less than £20 with postage, and have rebuilt the original door actuators ready for if/when they fail again.
Taking the actuators apart is fairly easy I guess, the motors are a push connection no soldering or anything like that, just take note of where everything goes and should be an easy fix.
I did a simple test on the old motors, just ran a 12v battery supply through them, 1 motor did not spin at all, 2 motors turned but not very well and, the last motor span fast and freely, so I think that confirmed the 3 motors had gone past their best.
Hope this helps regards Martin.