Interesting one on my 1999 4.6HSE.

Intermittently, the tailgate button stops working. Either it has no effect whatsoever, or occasionally (and this is the interesting bit) when I press it, the central locking operates and locks the vehicle! I unlock it with the fob, press the tailgate release button and presto it locks the vehicle again!

Has happened about 4-5 times now, and every time after it's been raining heavily - so I guess it's an water/electrical issue.

Have already drilled the tyre well drain hole so water is getting in.

I'm going to replace the button (with a chrome job) plus the actuator and lock. While I'm there I'll have a good look around the electrics.

Jock

Check out the front drivers side door lock/wiring as well.

-Wills

99% chance it's the driver's door lock assembly. My 2001 Vogue had
similar problems and a new lock cured all of them. It cost about £145
and took a good hour to fit but you will need a lot of patience. Good luck