No heat from passenger vents, but heat from windshield vents

On my '78 diesel Rabbit, I'm getting hot air out of the windshield vents but cold air out of the passenger vents when I turn on the heat.  I won't have time to really look at it until next weekend, but I did try briefly to remove the housing from around the heat exchanger to see what may be going on in there.  I haven't opened up that housing before and removing the two clips in front and screw at the top didn't seem to loosen it up enough to go anywhere.  Do I need to get the dash out of the way to open that up, or am I just forgetting a clip somewhere?  Without anything else to go on yet, does anybody have theories about what may be causing heat to only come from the top vents?

Thanks.

The side vents are a/c or Fresh air only, the Center Foot and Defroster are the only ones with heated air.

www.cabby-info.com has a mod where you insert a metalbestos tube cut some holes and have heat out of the side vents, but by nature what you are describing is "Normal".

If you have heat out of the center vents, then the cable isn't the issue as I don't know if you have an a/c equipped car as there is a difference in how the Center vents and things work there versus a non-a/c equipped car.

The A/C equipped car looks like this under the hood


It is controlled by vacuum servos and the Center vent servo operates by defrost as a default. It has a tendency to go bad, and leak, and you get no air blowing out of the center vents at all.  To replace it you have to take your dash apart well the Upper Dash out.


The non-a/c equipped is operated by a cable.

Thank you!!  This is my first winter with this car, so I didn't realize that this would be normal behavior.  And thanks for the info about the modification, but I will likely keep things as-is since I do seem to get enough heat when the front vents are off.  To address your other post, this car doesn't have A/C, but thanks for that info too.

Cheers!