When I started on the refresh of the Mini I found a lot of water under the carpet and in cargo area. I though that the previous owner took it for a swim in the lake.
:(
I’ve since replaced the door and hatch seals. Where I live, we don’t normally get a lot of rain but July as an unusually wet “monsoon” month for us. The amount of rain was high and it found its way into the hatch cargo area.

this led me to rip out the rear interior, again, but there was not clear point of entry. Not from Spoiler, not brake light and not from the handle or licence plate light.

That left the tail lights. I wasn’t sure the best way to address this but I have another car with a speed deployed wing and was able to eliminate water ingress with Dum dum strip caulk. So I used this, hope it stops the leak, will follow up again in a few weeks with the result.

 

Good luck!
Does spraying the car with a water hose reproduce the rear leak? This could potentially help to pinpoint the leak source if the strip caulk doesn't work out.

 

The hose method worked on the water leak for our F56 but the hose was not able to reproduce the leak in the R58

There is 0 evidence of where the water is coming from and we’ve had days and days of rain. Not a crap ton of water inside but enough to set me off to find a solution.

Will advise on the outcome after some travel and rainfall.

Any input is appreciated.

 

Check the seal on the antenna base just forward of the right rear taillight. When I changed the original antenna on my Coupe and had loosened the tensor nut inside the fender, I saw that the gasket was cracked and had lost most of its pliability. ​​