Thanks for clarification, on re-reading your initial query, I now realise that I was off the mark.
The paper trick seems like a good place to start. I'd also do the old water spray (use a hose) around the boot lid test, if you get water ingress, then providing that you have a 73/74 140, it may be possible to remove the tail lights and using a torch, shine a light inside the boot to watch for where the leaks are originating from, and chase accordingly. I have to do this on my 240, but i have been busy on a major COVID inspired garden infrastructure upgrade....
Good luck with it.