Does the microswitch under the panel click like it should? On these, the microswitch is held in with hot glue, and sometimes this can crack, and the switch moves.
If the switch is clicking, then listen for the actuator. This is an electronic motor pulling a rod which opens the lock. If this is not working, then it could be the actuator, the fuse or the wiring.
If the actuator is moving, then it could be the linkage between that and the lock, or something is jammed in the striker plate.
If none of these help, then you may need to check to see whether there is an emergency release in the boot trim (some cars let you slide a credit card or screwdriver through the panel in case of electrical failure). If not, there may be a manual key opener on the tailgate enabling you to bypass the electronics.
Pete G, December 2013
