Open the spare wheel well to give you some space. It will open if you force it.
Reach down with a 1p coin and force your hand between the tailgate carpet and the plastic hinge cover and with the coin, undo the 3 plastic trib screws.
Once they are undone, pull the trim off. It will come off if you bend it. Then you are in to the mechanism and can try and figure it out from there.
There may be an easier method and don't blame me if you break something but thats what I had to do last weekend when I'd been refurbing the button and put it back on wrong and closed it without checking it worked! Doh!
Nik