has anyone had this and what`s the fix?

when i open my tailgate after a carwash or rain, i get water stream onto the reat parcel shelf area. i always park on a kerb and its the side on the road/off the kerb that has it. It doesn`t matter which side i park on the kerb its always leaks in from the lower side. funny thing is that the water seems to collect midway down and then flows in.