probably a leak in the trunk. Or, with R32s, it could be the rear light clusters leaking as well - might want to give the seals a check.
The most straightforward way to check though is to grab a flashlight, and climb into the trunk and have your mate close it on you. Then, as he sprays water from a hose onto the trunk area, look for where it dribbles in. note: it's very awkward in the trunk and hard to move around much
