My rear doors to my 2008 gti hatchback wont lock, the front doors do lock, and the alarm is on and working, except that the back doors WON'T lock. What could it be?

2008 Saturn Vue liftgate will not open

The endgate/liftgate can be locked or unlocked by operating either one of the front door lock switches, your's doesn't do this ?
Endgate/Liftgate
Once the vehicle doors have been unlocked, the endgate/liftgate can be opened by using the handle mounted to the exterior of the gate. The endgate/liftgate can be locked or unlocked by operating either one of the front door lock switches, or by pressing the Lock/Unlock button on the key fob. The endgate/liftgate lock and unlock functions operate in conjunction with the rear doors.

The alarm going off is normal. Yes it will quit when it is fixed. Things to check first before you buy another door latch. First , unplug the plugs and plug them back in, see if it works. Corrosion make them loose contact. Next would be checking for power and ground at the plug. You will have to cycle the locks to check for this. If no power or ground. Then at the front of the door jam, you will see a rubber boot. It pops off both sides. This usually where wires break.

I would take a look at the sensor inside that rear door latch first.
Except for the windows, a faulty sensor would involve your other issues. Could be something with the right door module too.

bad main switch to windows and locks.

Hello! There is a Technical Service Bulletin # EL009-07 that addresses programing the door lock remote...Your dealer should have a copy...I mention the TSB up front as a possible solution to the problem...However I believe the drivers side Door Lock Assembly is defective or the wiring to the switch/motor is open...The assembly has a 4 pin connector...White/Red and White/Black are the switch contacts... Blue/Black and Blue/Red are the lock drive motor...If you have the expertise, gain access to the assembly and inspect the connector and test with a multimeter...If you are not comfortable doing that I would replace the assembly...

Guru............Saailer

I’m happy to help further over the phone at https://www.6ya.com/expert/david_6df67de3b14de867

If this is a factory alarm then possibly the driver's door power lock actuator is bad. 2008 Suburban may still be under factory warranty. Check with your dealer.

Check with auto parts stores for an aftermarket latch. Dorman Industries may supply one. If not, try for a salvage yard part.