To some this may sound petty, but engineers spend a lot of time on giving a car a "feel" to the controls. As all of you know, the resistance on the air vent dials is non existent. Sort of reminds me of Pinto quality. The good thing is, the design lends its self to a very easy fix once you have the panel off. You can find a description of how to take the vent panel off here at my Blend Door article Ultimate Blend Door repair & modification . Once you have the panel off, you need to install a total of 4 rubber O Rings as pictured, 2 on each vent. The pivot pin is an easy pinch to tip and push it out of the arm situation exposing the thimble for the O rings to be pulled over it and pushed down into the pictured position. Make sure they are seated snug up against the housing by pulling on the ring a few times all around so their pressure will be even. With the axel being thimble shaped, the two O rings all ways want to press down toward the brown housing, giving the perfect amount of resistance to the dial. The O rings I used were the R-09 ones found in the generic 419pc O-Ring Assortment pictured. Those sets can be had for about 12 bucks from a variety of online stores. The O ring in question measures at .500" ID and .100" thick. The thickness is important because any thing much thinner will wedge its self between the thimble and housing with poor results.
Just to add one more tip to those that go ahead with this, you might as well re fit the vent ports with new foam strips. To prepare the surface, carefully peel off the nasty sticky strip that has the rotting foam that's pictured above. Then use a thinner or acetone to rub out the area where the sticky strip was until its perfectly clean to the plastic. I use some basic stick on household weather striping from Lowes that's pictured. It wont stick good unless you do a good job cleaning the plastic with the thinner. I used medium paint reducer. It comes out pretty good as you can see.