there is a rod each side of the lock/handle that runs to the catches on each side of tailgate , this pushes the cath to unlock it on each side . .
behind the button you push to open the tailgate is a plastic pin which pushes a swivelling pivot which makes the rods move .
you may be able to knock off the riveted steel cover plate inside the vehicle thats behind the handle/lock and then you can use a screwdriver to move the pivot so that it releases the catches each side of tailgate top .
the rods run through the channel at bottom of tailgate and are not accessable .
its more than likely that rust inside the holes at the catch ends of the bottom of the tailgate frame wont allow the rods to move and release the cathes and its just bending the rods when you push the release knob in the handle .
the rods PUSH out to release the catches and you cant get at them or release them by prying the actual catches, you may release the catches with a pry bar but i doubt theyll work again .
concentrate on the cover plate behind the handle and then you will access the pivot plate and be able to pry it and then puish the rods to release catches .
lots of WD40 is reqd also .