The year, make and model would help but it sounds like that would be part of the HVAC (Heating, Ventilation, Air Conditioning) duct work.
The silver housing is mounted in the engine compartment, and the black housing is mounted under the glove box in the passenger area.
Its under the cowl in engine compartment (close to windshield) in one of the heater hoses that leads to heater core. Black and white plastic with a small vacuum hose attached to the top.
passenger side of engine compartment by the firewall on top of the black housing for the blower motor. 2 screws hold it into the housing
On a 1998 Windstar the vacuum reservoir tank is under the hood in the back left of the engine compartment when standing in front of the vehicle. Mine has a red and black vacuum hose coming out of it.
Could be the vacuum canister yes, could be
That black ball is called the vacuum reservoir. Its actually attached to the air filter box and is used primarily during acceleration
The lvl select door is vacuum operated. I'm not sure where the vacuum block is located on this year, but if u look in a manual it will help. You should see the vacuum lines under the hood on the firewall. There is a black housing that they connect into. Replacing that housing should fix the problem unless you have a cracked Vacuum line. You can get a cheapy T shaped line at most parts stores.
Defrost is the default position. Most likely a vacuum leak somewhere between the engine compartment and the passenger compartment. Look for a small black hose going through firewall ti the heater box inside. It should have vacuum from the engine on it. if not it's on the engine side. if it does it's probably in the passenger compartment. just trace the line back looking for the leak.
The compartment is for the storage of cleaning patches.
A vacuum and a black hole are two very different things.
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