I think you will need to attack the drains from the bottom of the car.
I will see if I can locate the rear drain outlet and take a picture.

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Current: '10 E70 X5 35d
Prior: '82 E21 320i | '80 E21 320i | '72 E3 Bavaria | '98 E36/5 318ti | '06 E46/2 325Ci | '83 E21 320iS