air door location and how to replace

This is normally always the mode door actuator. It directs the air flow to the different positions that you select. Here is a guide that will help with this:

https://www.2carpros.com/articles/air-vents-stay-in-the-defrost-position

Your vehicle actually still uses vacuum diaphragms for the floor/defrost door and a separate one for the panel door.

Below is the process from the manual that will help with replacing them. A lot of times the line just comes off the actuator so that may be all the issue is.

Please let us know if you have other questions. Thanks