I believe my car does have two heater pipes. One is the pipe you are thinking of, which runs across the motor and behind the fan, and I have another which runs under the manifold to the back of the engine. It appeared to be a return pipe from the heater core. Anyway, on my car that pipe was bolted to the bottom of the intake runners, and could only be accessed from under the car with about 2.5 feet of socket extensions. It bolted to the driver's side of the water pump with its own separate o-ring. This is on a coupe, and was the first year for the w104, so it may be different than other cars (which may also explain the fan situation).