T5 trident rear door

Assuming you mean the sliding door and not the tailgate - yes, we had a similar problem on the Trooper. In our case the cause was stretch to the inner of the cable that runs to the latch at the back. Unfortunately you cannot buy just the cable from VW - you have to buy the complete lock/latch assembly, which from memory is well over £200!

I fixed ours by making a small extension to fit on the catch release lever - but it's not something that I could have done away from home.

You can diagnose the issue further if you can get the door panel off - which is not easy if the door won't open. We were lucky with ours because it got to a stage where it would open one in maybe 20 goes, right on the limit of the inside handle movement - we just kept trying until it opened and then immediately took the interior panel off. Not much fun if you're away, as from memory you have to remove the upper surround panel and blind too...