First post gang!
Just picked up an '06 330xi and it has leaking WW fluid. Leak is at same location as OP, side-skirt/fender seam between passenger door and wheel-well. Opened up the wheel well and pulled the reservoir to where I could see both pumps and it was the windshield pump housing that is leaking. I drained all the WW fluid from the tank and put it in a gallon-size bottle. I removed the pump and pumped silicone into the crevice where it was leaking. I reinstalled the pump and after 24hrs will refill the WW tank and see what happens! Will report back in a day or two with results. If the silicone doesn't work, well then a new pump is my future.
UPDATE: Didn't put the WW fluid back in for about 3 days, but hasn't leaked again after using the WW a couple times. In the past, it would leak after each use. So, fingers crossed that the silicone sealed the crack or whatever the leak was in the pump. I have some pics of where the leak was if anyone has similar issue and wants to see what I did.