The first step for vacuum-element repairs is to get a Mityvac. As RR has said, it is likely that there is a bad actuator, but it needs proper diagnosis. If the Indy has properly diagnosed it, you're in for some work.

The footwell flap is accessible through the radio opening, I've done it, it's possible but painful. The '88 has one rectangular element for the footwells (late '87 up).

Defrost I believe is the one on top of the unit, but if the defrost vents are open, it is not your problem, it will fail open if broken (or no vacuum).

If you get defrost only on all settings, it is the diverter flap and/or others such as the footwell flap that fails closed.

The vacuum-control unit is just to the left of the glovebox (or pass airbag). You can disconnect the vacuum lines one at a time, and hook up a mityvac to see if each pod/chamber holds vacuum (and you can hear most of them move).

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Gone to the dark side

- Jeff