Looking for information regarding the vacuum system mentioned in the first 2 reply's.
Ford uses plastic lines for the vacuum system and over time they can deteriorate. Plastic gets brittle over time and can also deform or melt should the temperature get to high.
At the front Right of the engine, look for a block with White, Red and black lines connected to it. One of them goes to the Vacuum reservoir from the information I have been able to dig up.
Should one of the vacuum lines fail the blower system would go the default Brett mentioned.
With the loose of vacuum you should notice a difference in the performance of the engine. If you have access to a code reader, there should be a code set, that indicates loose of vacuum.
If you have a check engine light, the loose of proper vacuum could be the cause of the check engine light.