[oldskool]

I havent had my rear lights out yet ? Is there a rubber seal then that gets compressed when you tighten the light in ?

Coming from lr background ive heard a lot about water ingress

Ive used spray sillicone and normal grease on door/boot seals before and its done the trick

If theres a seal kinda fitted to metal as well as spraying or greasing the part of seal thats visible its always worthwhile to pull the seal off and grease/sillicone spray the lip of metal and mounting face if the seal so water cant get in by going under the seal if that makes sense

Best stuff for sealing things up that you are sure you wont want to take apart is sikoflex ... It doesnt leak but is near impossible to take apart again if used

Captain tolleys creeping crack is another product people use , ive not used it but many have and its supposed to be great ,, afaik its real thin and you pour it where roughly you think the leak is and it finds the gap and seals it

Ive used ordinary clear bath sealant before to test if an area is leaking and seal it to test ,, easily removable