So, just behind the door skins is the yellow plastic sheet that Mercedes calls "door covering." The sheet directs water away from the door skins and down to the drains at the bottom of the door. (Yeah, there are slotted drain holes at the bottom of the doors.) The front pre-cut sheets 201 727 00 87 are a mere $4 per side.
However, the rears, which are smaller, 201 737 00 87 are . . . $15 per side.
A trip to the hardware store for some plastic sheeting and a new #11 blade will solve that issue.