The vacuum element for the defrost flap is mounted on the side of the blower box behind the glove box, which is easy to remove. The default position for the flaps is defrost on, so if the rubber diaphragms in the element are shot the defrost will be open. Pushing the EC or chill buttons should close the defrost flap (although it's a little more complicated than that, which is why it is a two-stage element). The part is $34 or so in Fastlane, a little more at the dealer. You can snap the element out, pry it open (springs and stuff inside) and check to see whether the diaphragms are torn. Easy diagnostic, although look for more, less accesible, elements to fail, too.

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Jim Wiggin
Columbus, OH
1989 300E