My 96 Park Ave vent doors do not open, air blows out defrost and floor only. Any suggestions as to a fix?

Sounds like there is no vacuum. Check in the engine bay for a vacuum line not connected to anything.

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or cracked, or the connection at the back of the control, or at the vent

Moved to proper section.

Check the vacuum line that runs to the engine and looks out of place. Do you have the 3.8? If so..it'* right on the back of the black plenum next to the big brake booster hose. Taht little hose must have vacuum in your year to make the vacuum controlled doors work.

Thanks, I'll check there.

No vacuum line to operate acuator doors on this model, all doors are positioned by a DC motor. You need to run a scan on the system. I'm told that you can get this done at Autozone for free or you can do it your self by pushing the OFF,AUTO, AND RECIRC buttons at the sametime with the ignition key in the on position and the engine not running. you'll get a code number to lookup in the DTC (Diagnostic Trouble Codes. You fine the codes at: https://www.gmforum.com/1992-1999-91/1998-dtc-code-listing-w-descriptions-inside-196770/

Replace the HVAC control unit (the module that houses the display and all the controls). Shouldn't be too hard to find a used one at a reasonable price.