several things. vacuum reservoir, vacuum ball. It is located under the passenger side fender.
Mine was shattered when i hit a tree and I had to replace it because, like yours, my air was stuck on defrost. I searched for days trying to find an aftermarket source, I finally bought one from a memeber of the forum. there are two hoses going into the ball, the ball provides a seal and pressure, if cracked, your air is stuck on defrost