Check/replace fuses 23 (10A) in the main underhood fuse panel. Then fuse 1 (40A), and fuse 8 (20A) in the aux underhood fuse panel. Then fuse 10 (7.5A) in the underdash fuse panel. Operaytion of the system relies on secure ground connections for the controller, the sensors/switches, and the gate actuator. G503, G610, G602 ground points need to be intact and secure. See the FSM for the locations of those; trying to describe them to you is not nearly as effective as having you look at the location pictures.
The power liftgate controller lives in the left rear wheelhouse area, above the trailer interface controller if you have that. The interior panel needs to come off for access.
Diagnostics typically start at the MICU and a communications test (requires HDS) to make sure the liftgate module responds to CANBUS commands.
After that, the diagnosis proceeds to external sensor tests on that liftgate control unit. You get to unplug the controller and test each input signal via those harness plugs, using your DMM. The whole test procedure starts on page 22-656 in volume 2 of your FSM.