Perhaps I can help all of you to find your vacume leaks.

There are seven vacume motors in a 200 and here is a list and what state they are in.

The floor heat switch operates four motors. When the switch is in the "off" position, the motors it controls are in the following state.

The two vacume motors which control heat to the back seat (one for each side)vents do not have vacume and are closed. They are located directly below and slightly behind the vacume switch cluster.
The two vacume motors which control heat for the front seat (one for each side) have vacume and are closed. They are located direclty above the vent openings.

Since any hissing in the switch area could be the back seat motors, hose or tee to the back seat motors, or a loose or broken hose on the switch itself, turning the heat off with the floor switch will remove vacume from the back seat floor motors. If the hiss disapears, there is your problem. If not, it is a leak around the switch.

When the defrost switch is in the "off" position, the defrost vent motors do not have vacume and are in the closed position. The motors are located directly below the defrost vents.

The last motor is the fresh-air/inside-air door. When the switch is in the fresh air position (off), the motor has vacume. It is located in front of the fresh air vent door.

By switching the switches and listening for the change in hisses, you should be able to figure it out.

The front floor vents and defroster vent motors are pretty close together so you may have to operate the switches to find it.

The vacume reservoir is located beneath the heater/AC assembly in the center console. Looks like four plastic balls glued together. The vacume enters on the right side of the reservoir and exits on the left side by the driver's right foot near the gas pedal behind the kick panel. This hose goes straight to the vacume switch.

The main supply hose comes to the reservoir, through the fire wall above the passangers foot area, and there is a vacume check valve located right before the vacume port on top of the intake manifold.

On the switch, there are two hoses for the floor, one hose for the incoming vacume, one hose for the defrost and one hose for the fresh-air door. There is a tee below the switch for the back seat floor vents and two tees on top of the blower housing. One for the defrost vents and one for the front floor vents. The hose for the fresh-air door goes over the top of the blower housing and then direct to the door.

Hope this helps.

Gambit...