Why does the floor of the rear passenger side get wet after it rains on my 2002 BMW 525I?

Either someone keeps wetting themselves in the back...

Or your rain seals are not sealing. this could be at the sunroof or the tops of the doors.

Check they are fitted correctly round the doors first.

also parking on a hill or incline wont help.

a car cover is cheaper than fixing a sunroof seal.