The problem when this occurs is the lock actuator in the rear liftgate. The interior panel does have to come off to replace the actuator. Not impossible to do, but it is harder with the liftgate closed. Remove the pull handle first, then remove upper portion by prying away from door, then lower portion.
The rear actuator works off the same circuit as the rear doors. If the problem were in the relays or wiring up front, the rear doors would not lock or unlock as well. Replaced lots of actuators on these SUVs for the same symptoms, so I am pretty certain that is what is going on with yours as well.
Trim Panel Replacement - Lift Gate
Removal Procedure
Remove upper trim panel.Pry gently with a thin-bladed tool in order to release the clips.Slide the trim panel upward to disengage mounting hooks.Remove the trim panel from the liftgate.Installation Procedure
Position the trim panel to vehicle.Slide the trim panel downward to engage mounting hooks.Install the upper trim panel.Install the pull strap.Once you are in there, you should be able to see the lock actuator and also how it works. I think it is fired electrically of course and then a cable pulls on the lock.
Trim Panel Replacement - Lift Gate
Removal Procedure
Remove upper trim panel.Pry gently with a thin-bladed tool in order to release the clips.Slide the trim panel upward to disengage mounting hooks.Remove the trim panel from the liftgate.Installation Procedure
Position the trim panel to vehicle.Slide the trim panel downward to engage mounting hooks.Install the upper trim panel.Install the pull strap.Once you are in there, you should be able to see the lock actuator and also how it works. I think it is fired electrically of course and then a cable pulls on the lock.
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It just says to unplug the lock actuator from its wires and remove the bolts that hold the acuator on.GM doesn't just sell the lock actuator by itself. You have to purchase the entire bracket and wires that the actuator sits on, which is more than $300 for a part
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There is basic troubleshooting for this problem, if not done, you can follow:---
A key remote may be used to unlock and lock the doors and liftgate automatically. The ignition key may be used to manually unlock and lock all doors and the liftgate. The liftgate may become stuck and difficult to open if the vehicle isn't properly parked or there is a buildup of dirt in the locking mechanism.----------