Mine doesn't have any kind of 'chimney', but if you look closely on the right side of pic #1 & #3 of post #134, you will see part of a piece of 3" white vent hose sticking out. That went from one of those chrome vents on the outside to a round register mounted next to the A/C vents. The only thing I can figure this was used for, is to allow fresh air to enter the patient compartment when the exhaust fan was operating in the left rear top compartment. This would draw air across the patient and out the back.
The box the A/C evaporator and blower is mounted in is basically sealed, with the intake on the side just above the walk-thru door, and the discharge through the two groups of adjustable vents. That means the only way all of those diesel exhaust particles could have gotten in there is to pull them out of the inside air. Also of note, the intake vents do not have any kind of filter or system to stop anything from going through the blower or into the A/C mounting box.
The blower discharge was not actually connected to the vents in any way either. It was just sitting behind the vents, so any dust or particles would be forced into the box, as well as recirculated back into the patient compartment. Since the patient compartment gets cleaned and wiped down after every run, no visible build up ever occurred, however there is some now. The A/C compartment is not accessible to the crew, so it never got cleaned.
A replaceable intake filter like your home A/C unit has would have solved the problem. That design is piss-poor if you ask me, and the fact remains the patient compartment was not sealed properly to prevent exhaust from entering it.
The undercarriage is covered in the same oily black exhaust residue. And that boys and girls, is one reason why I'll never own a diesel.