Vitor is probably right. You will still have to take a door panel off and then using your fob, watch the relevant parts work/not work. It could also be a relay or two, but don't ask me which ones.
Klaus
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98 V70Rawd(108Kmi), 95 854T(88K mi), 75 164E(173K mi)

Let us isolate the problem first. Make sure the electrical lock mechanism works first before we look at the remote.

Use the inside driver arm rest unlock lever to test. Put your ear close to the locking door mechanism to the door in question. You should hear the locking motor when you flip the lever both ways. If the door either locks or unlock, we know we are getting current to the motor. If you are able to lock, but not unlock, then it is most likely the dreaded REPL. When you press the unlock button and hear the motor turn but the door is still locked, the REPL is most likely the culprit. I had to replace both left rear and right rear mechanism. First I bought the left lock mechanism and later tackled the right using Vitor's instructions.

Now if you determined the above works, and when use your remote and the locks don't work, then it is most likely something else and not REPL.