does anyone know what could cause it? its only showing up to the right of the battery

 

Usually the sunroof seal or the hatch seal will allow the water to creep in. IMO find it before it does damage to the electric of your MINI.

Do a search as there were numerous threads on this.

 

Also look at the seal around the 3rd brake light and the foam seals around the rear brake/turn signals....

 

I just diagnosed and fixed this problem with an 06 Cooper S that we purchased. I suggest the following to pin-point the problem.

- Remove all plastic panels from the inside of your hatch
- clean and dry all parts
- hose down your rear hatch and roof while someone is inside (seats folded down) to watch for drips
- stop hosing, inspect for drips and puddles
- target problem and fix with OEM clips and seals from your dealer.
- repeat hose down procedure
- put plastic panels back if no leaks found

Common areas as stated above are the rear hatch handle gasket, third brake light gasket, and as was the case with our R53 the holes which house the clips to hold on the trim immediately below the rear window glass.

I purchased 9 new clips and prised out the old ones. I placed a liberal dab of black silicone around each clip head before snapping them into place. Repeated hose-down of vehicle and no leaks found.

This will take a bit of hunting and patience.

 

Also IMHO sunroof seal and rear tail-light cluster leaks are more prone to Volkswagens - these are less likely on your Mini.