The tailgate release is controlled by the convenience control module, located behind the glovebox.
I've attached a photo showing the module and the location of the tailgate release wire. To force the actuator, unplug the black release wire connector and use a short link wire to bridge the release wire to the 12V power supply. If the tailgate release activates you know the wiring between the actuator and the module is all good, meaning the control module isn't sending the release signal for some reason.

As well as the release actuator, there are two switches inside the tailgate latch. So the actuator and two switches accounts for 3 of the pins, then there is the earth/ground on the 4th.
You can use the rear wiper and boot light to test the switches inside the latch. Make sure the boot light switches on & off when you open & close the tailgate, and make sure the rear wiper DOESN'T work when the tailgate is open.

TAILGATE RELEASE - RED/BLUE WIRE

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