there are any differences between your '79 and my '85 but on the '85 you remove the cover underneath the steering wheel and look to the right of the gas (fuel?) pedal. The one closet to the floor is for the floor vents, the one closest to the windshield is for the defrost vent.
Underneath the pass. side cover and behind the glove box are the recir/outside damper actuator (the one closest to the side of the car and windshield) and the "main" damper actuator (which is staring you in the face when you remove the glovebox). The actuator for the center vents is behind the center vents under the center of the dash. I think the dash cover has to come off to replace it. You CAN see it if you look real hard from the pass. side feet area.

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Jim