Car is a 2004 Cooper S, new to me. I replaced the battery, cleaned it up well. It did look dirty and rusty in the battery area. After replacing the battery, after being out in the rain, I saw water in the battery tray, about an inch or so. Cleaned it all up of course, but… any suggestions on how to track down the cause?

 

There are a few possible entry points to check:

3rd brake light
hatch handle
trim strip

Best to do a test with hose if you have a helper to hold it in various spots while you inspect to try and find source

Also a very good idea to unplug the drain hole in bottom of battery box --- it is there but covered over at the factory for some dumb reason, you'll see it when you remove the skid plate thingy

 

Mine turned out to be the 3rd brake light and to a lesser degree, the passenger tail / brake light. Try gently and evenly tightening the 3rd brake light screws; they seem to loosen up over time.

 

Thank you, I’ll definitely try that. Much appreciated.