I was thinking of how to see physically. Presumably it is visible - I think that others who have reported your problem over the last few months have been able to see the evidence without serious dismantling.
Robin, if you're not used to replacing grommets, I'd confine yourself to checking the exterior of the visible rubber. They can be a bit fiddly to put back & if they're not replaced perfectly, you'll be joining the "I've got a puddle in the rear of the car & I don't know where it's coming from" thread. Also, the wiring inside the grommet is double wrapped in cloth & looking at it won't tell you much. I'd expect the outside of the grommet will be the first thing to fail.
Check behind the visible rubber boot (the surface that's against the bodywork when the tailgate is shut) for signs of wear. You might find some of the paint has rubbed off, and there could be a corresponding area of eroded paint on the rear hatch frame.
I've posted some pictures to show what you're looking for, and also one where I've removed the grommet that fits into the bodyshell to show the cloth-wrapped wiring inside it. Click to enlarge - I've left them in full resolution so you can see the detail.
This is a monumental piece of crap design & I'd say the only real question is when it's going to fail. Now that I've checked mine, I'm going to have to think up a fix & also touch up the eroded paintwork - it's actually rubbed right through the primer to the bare metal
.
I wouldn't be surprised to find the grommet is part of the loom & replacing it properly looks like it would be a nightmare. The big problem with any workaround will be finding a way of preventing water from leaking into the loom, the tailgate & the rear of the bodyshell
. Also, once the rubber boot does split, water will once again compound the problem & it won't be long before electrical troubles begin in earnest
:bang::bang:.