The problem was a blown out rubber diaphragm inside the Vacuum Element - Main Air Flap Control (the one on the right side when looking at it from the passenger seat. You can access all three of these vacuum valves after removing the glove box insert.

Once you remove the valves, either disassemble them and inspect OR plug the holes and see if they hold vacuum after pulling / pushing the rod.

Ordered Part #: 000-800-57-75-M22 from Pelican Parts.

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1987 420 SEL | 87k miles
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