Mystery oil leak

A G owner (460 300gd) called in today he was a french guy doing business and had been sent to me by the local garage.
His problem, the 300gd was loosing oil and it was dripping down the inside of the gearbox cover onto the carpet and his shoes.
First to isolate the leak that was not difficult, the rocker cover gasket had perished, a rubber gasket and not difficult to replace, but how was the oil getting into the car after all the G is water tight, I steam cleaned the engine bay to get rid of all oil and paid particular attention to the bulk head, ran the engine in the worshop, no oil, drove down the road, oil dripping on the carpet as before, back to the workshop up on the ramp look underneath,
Like mine the 300 had air con and there is the clue. the drain tubes from the condensor had on a previous clutch replacement been fastend to the gearbox not to the locations on the tunnel, result when the oil leaked it ran back down the gearbox over the drain tubes and was drawn up by the vacuun effect of the air flowing through the vehicle, result the air con box had gradualy become saturated with oil until it over flowed back into the vehicle.
The french man left with grateful thanks to me and many strange french words about the guy who fixed his clutch whom I have a feeling will now be finding out how to strip an aircon box.
Moral of this story. allways put it back the way it came apart. MB spend a lot of time getting little details like that right, and little changes can have irritating effects.