Cannot unlock and open rear cargo door with key or electric lock

Service repair info. manual would be the place to start ,seeing how you have no clue .,or maybe a qualified repair shop. How are you going to fix it with out knowledge of how it works an what to check , --------------------------------------------------------------------------------
Power Door Locks Circuit Description
The power door locks are operated by reversible motors. The motors are found in the following locations:
• In each front door
• The side door
• The rear door
The PWR ACCY circuit breaker (30A) supplies voltage to the door lock motors.
If you move either power door lock switch, mounted in the front doors, the LOCK applies voltage to cavity F of the door lock relay. This energizes the lock coil in the relay. Current then flows through CKT 295 (GRY) directly to the following components:
• Both front door lock motors
• The side door lock motor
• The rear door lock motor (cargo doors only)
CKT 246/294 (TAN) provides the ground for the motors. The normally closed contacts of the UNLOCK then contacts the door lock relay.
If you move one of the door lock switches to UNLOCK the polarity of the system reverses. Current flows through the energized UNLOCK relay contacts. The LOCK relay contacts provide ground.
When the UNLOCK button is pressed once on the RKE transmitter, only the driver door unlocks. If it is pressed a second time within 5 seconds, all doors unlock.
If the side door is open, the side door jamb switch ground signal is opened/broken. If you move the door lock switch to LOCK while the side door is open, the following actions occur:
• The 2 front doors lock
• The rear door locks (cargo doors only)
When you close the side door, the door jamb switch closes. This closes the ground signal to the body control module {CKT 775 (PNK)}. A 5 second timer then starts in the module. After 5 seconds, cavity A4 connector C1 of the body control module supplies voltage to the door lock relay via CKT 195 (LT BLU). This energizes the door lock relay, which sends voltage to the door lock motors in order to lock the side door. Refer to Door Jamb Switches, Liftgate Strut Switch for a schematic of all door jamb switch inputs.
If the side door opens during this 5 seconds, the 5 seconds timer is reset. The locking pulse takes place at the end of the 5 seconds. Unlocking the door resets the 5 second timer in the body control module.
Use the following components in order to unlock the door lock system:
• The LH power door lock switch
• The RH power door lock switch
On vehicles equipped with cargo doors, the cargo door opener mechanically opens the door once the door lock system is unlocked. On vehicle equipped with a liftgate, the liftgate lock/unlock release switch (mechanical key) is added in order to serve the same function as the front power door lock switches. These functions locking and unlocking all doors, including the liftgate. Once the door lock system is unlocked, press the liftgate release switch in order to electrically open the liftgate and pull the handle in order to open the secondary latch.

door handle latch could be loose, but kind a weird happend on booth doors, souds more like child lock protection: Child Protection Door Lock Location
To use the system, open each rear door, use a flat blade
screwdriver (or valet key) and rotate the dial counter-clockwise to engage the lock and clockwise to disengage
the child protection locks. When the system on a door is
engaged, that door can only be opened by using the
outside door handle even if the inside door lock is in the
unlocked position.

rear door hatch lock repair GMC Chevrolet - YouTube

? 4:26
https://www.youtube.com/watch?v=QS3PMhGgyAg
Apr 5, 2014 - Uploaded by auto resource
2002 Yukon, rear hatch lock not locking. ... rear door hatch lock repair GMC Chevrolet. auto resource ... chevy tahoe lift gate notopening - Duration: 2:12. ... A Method how to unlock your car in 10 seconds :) - Duration: 0:35.

chevy tahoe lift gate not opening - YouTube

? 2:12
https://www.youtube.com/watch?v=xU6ybiY3F4I
Sep 1, 2010 - Uploaded by 76bluemoon
Back lift gate/glass would not open or release. Even when unlocked with the key fob or the unlock switch on the drivers/passenger door. ... How To Install Replace Rear Lift Gate handle 2002-09 GMC Envoy Chevy Trailblazer ...

pull the cover off and operate it from inside

key is manual lock and electric assisted unlock . charge the battery with red screw on fuse panel Use key to unlock turning to rear.
Place key in ignition then turn on and off 8 times leaving it in on position until it beeps approx 5 sec then press any button on the key and turn ignition off.

Have you checked the fuses for those circuits ?

Power Door Locks Circuit Description
The power door lock system incorporates door lock switches on both front doors and on the RH D-pillar, door lock motors in all passenger doors and in the RR cargo door, a door lock relay and a door lock control module.
Power for the door lock switches and the door lock control module is supplied from the CIG LTR fuse 13 through CKT 640 (ORN). Power for the door lock relay is supplied from the PWR ACCY circuit breaker through CKT 540 (ORN). Ground for the door lock relay is supplied from G202 through CKT 150 (BLK) and ground for the door lock control module is supplied from G400 through CKT 150 (BLK).
The door lock motors are reversible and both sides are connected to the normally closed contacts of the door lock relay. Those contacts of the relay are connected to G202. When any of the door lock switches are placed in the lock position, voltage is supplied through CKT 195 (LT BLU) to the relay lock coil. Since the other side of the lock coil is connected to ground, the coil is energized and it pulls the lock contacts closed. Voltage is then supplied through CKT 295 (GRY) to all the door lock motors and the doors lock. The unlock function operates in much the same manner except that when any of the switches are placed in the unlock position, voltage is supplied through CKT 194 (WHT) to the unlock coil of the relay and CKT 294 (TAN) supplies voltage to the apposite side of the door lock motor and the doors unlock.
Door Lock Control Module Description
The door lock control module is used on vehicles equipped with cargo doors. The purpose of the module is to lock the cargo doors after a lock cycle has been commanded from any of the door lock switches with the cargo doors open. When the RH cargo door is open, the module receives a ground signal from the closed contacts of the rear door jamb switch on CKT 1897 (ORN/BLK) to G400. When a lock command is received, the module retains this input until it receives an open input from the rear door jamb switch. Then the module sends a lock command through CKT 194 (WHT) to the door lock relay and the doors lock.
Rear Cargo Doors Description
The power door lock motor will not operate unless the rear cargo doors are fully closed. The power door lock motor is in the right cargo door. The contactors are on the rear cargo door opening on the body and on the right cargo door.

the electric door lock button should unlock the rear lift gate.

Hi! Yes its possible if manual you want it to be. Try this, If the cargo doors have separate fuse from the fuse box then just remove the fuse and it would work manually. If you wont find the door fuse then you should work on the door manually. Open the cargo door trimming and you will find a motor inside the door. The motor has a connector that leads to the lock. Remove the connector to cut the power supply to the motor and you'll be on manual now. Hope this helps and have a good one!

your lock acuteuator is stuck replace it and you should be fine