Whilst giving my E86 a proper clean this morning I noticed my battery was sitting in a puddle of water. Annoyed I started removing carpet and trim until I could find where it was coming from. The carpet was bone dry so I didn’t think the boot seals were to blame.
I eventually found the issue. There is vent on both sides between the rear wheels and light clusters. The rubber seal on this vent was completely perished.
Here is pic showing the location of the vent (already removed). This is the drivers side (UK) and right in the internal corner of the car (would be behind the bumper from the outside).

And here is the removed vent and damaged seal.

I’m going to order a new one and hopefully that will cure the issue.
I eventually found the issue. There is vent on both sides between the rear wheels and light clusters. The rubber seal on this vent was completely perished.
Here is pic showing the location of the vent (already removed). This is the drivers side (UK) and right in the internal corner of the car (would be behind the bumper from the outside).

And here is the removed vent and damaged seal.

I’m going to order a new one and hopefully that will cure the issue.