Take a good look at the manual lock - any signs of corrosion?

Our first 745 had a protective lock flap that was bent - as a result, water and salt got in and the lock would not work with the master lock.

I had to remove the lock and clean and lube all the lock parts - I could not save the tumblers.

The master lock did work however, and I epoxied a penny over the key slot.

If your rear hatch is like the 850, you remove the surround from the manual latch ( I broke my surround - good luck. ) I think that there are some fasteners at the bottom of the tail hatch, after you remove them you can lift and pull the panel away from the hatch.

The upper fasteners are interesting - you remove them by lifting them out of a slot, but to put the panel back, you install the fasteners in the slots and then push the panel onto the fasteners.

If the plastic is damaged around the fasteners, there is a metal kit to repair and strengthen them.