Problem The rear hatch door will not unlock.
First, on the rear hatch, there is an actuator. On FX, when you close the trunk, it kind of **** the hatch closed all the way. When you press lock on the remote, this same actuator locks it as well.
1) when you unlock the car from the inside, go back there and press the button to open the hatch door manually, do you here any noises?
2) Close all doors, go back close to the hatch button and using the remote , lock the car and then unlock the car. you should hear a sound from the actuator locking and unlocking the lock on the rear hatch!
If you don't hear this, your actuator is inop.
Next step would be to check the fuses inside the car, on the left bottom driver side, and outside on the battery compartment.
It might be just a fuse.
If all fuses check ok, your actuator is most likely gone! Not ea expensive fix, just time consuming!
NOW, if you can hear the noises, then something is either broken inside the actuator or one of the cables is loose!
Hope that helps.
Also, what year and miles on your fx?