Just came across this thread whilst looking/replying to a similar one (hence why it popped up) but thought I'd add my 2 penneth. As above I also have a 2022 T6.1, but mine is ply lines AND insulated and carpet lined. I did the insulation, and sound deadening myself (as I wanted it done properly, before handing it over to a trusted supplier for the carpet lining) and I did the tail gate to death. (see some example pictures below as I photographed my progress to prove to any new owner what I'd done). Anyhow, my belief is that this is caused by condensation build up over time. Agreed, removing the plugs let this drain away, but my OEM fibreboard tailgate liner was always really cold with the odd bit of surface condensation on the interior face in the really harsh winter. This year, during the cold snaps, of which we have had plenty, I've had absolutely no issues at all. My previous T6 and T5 which were 'proper tradies plylined' vans were always damp, especially on the roof, during the cold/freezing winter months. But as I say now, this T6.1 after my efforts has been sorted and no complaints.
So... might be worth people who are having this issue, whipping the tailgate cover off and spending an hour or so insulating the cavity. Appreciate this may not be 'your' issue as there may?? be genuine external ingress, (if I recall the H/L Brake light on my T5 was a source of frustration for ingress) but I certainly feel the cold damp void does not help.
As a side note (which made me think of this), I have a '23 plate MX5 that I own that lives on my driveway. The boot lid on these does not have an internal cover (just bare painted metal), and I get exactly the same issue in the winter months. open the boot, covered in condensation which then runs into the boot lid cavity when you lift it. I'm going to do a similar thing to this as per the 'Bus' to remedy.
just my thoughts/take on the issue??????
