Here are some pictures from the area of the centre side frame, below the air grille (note the rusty surface - which seems to be stronger at the picture than it really is).

The problem is that water coming through the air grille is piped into the side rail. After re-piping it through a (new) 8mm-hole which I drilled under the rear seat (pic. 2), I waxed all the hollow space with a special rust protection wax, thus hoping to decelerate the rusting-out of the body.

For the re-piping I shortened the original hose by about 4 inches, installed a reducer and used a new, smaller hose.