You went about the reverse light the correct way, when you had the trim off there is 3 x 10mm nuts to undo then remove the complete light from the outside of the tail gate.
The problem with the attessa sounds like it could be the accumulator, this causes the system to continually pressurise as the accumulator has lost its stored pressure. A new accumulator from Nissan is $350-400 plus fitting if you can't do it your self.
I checked out your wagon while it was at Vinces, looked good.