I have read a number of threads about tailgate release switches "furring up" and the problems that are caused. That may well be the reason for my (slightly different) problem, Previously, unlocking with the remote unlocked all the doors and the tailgate, then one day the tailgate didn't unlock. It can still be unlocked with the tailgate button on the remote, so no great hardship, but I wondered whether anyone else had experience of this?

It will still be the tailgate switch contact affected by water. The remote opening of the doors and tailgate UNLOCKS only (hand needed to press switch to open), whereas the remote tailgate opening UNLOCKS and OPENS the tailgate (pull up anywhere to open).
Hope that makes sense

remove the tailgate handle and the the rubber that covers it,clean it and try that if still doesnt work strip a small amount of insulation back from the two wires comming from the switch plug it back in and then connect the two wires either by holding together or with a small piece of wire/metal this will do what the switch is meant to do,if you hear the release working then you prob have a broken wire behind the switch......i had the exact same issue and found that one of the wires was broken at the switch...if it is broken at the switch you will need to dig out all the bonding that insulates the switch..bit of a headach but a reasonable easy and cheap fix.

Thanks for all the advice - if it ever stops raining down here, I'll give it a go.
Just an update - it was an easy job, cleaned white deposit from everything (not too much), but it didn't solve the problem. A £23 switch from the dealers cured it!