I'm currently on a mission to fix a leak in the trunk of my UR. Every time I park in rain the spare wheel well fills with water. I have the trunk stripped out and so far I have eliminated the trunk opening and seal and the rear side and back windows. (I hope)  The only other source I could see would be the tail lights. I decided to reseal them though I could see no sign of leakage and decided to use Bostik 70-03 that I've used before for similar purposes but on reflection decided against it as it may be a bit too permanent if the lights needed to be removed sometime in the future. The lights were sealed with some black non-hardening caulking that didn't look original. Has anybody got some ideas of what to use?
The parts book shows "Sealing cord in coils of 10m" and  "no number" which I guess is years NLA.

Went with RTV