Update:
I attempted to do the same routine that Michael did. Although accessing the area went without a hitch, it was not where I should have been looking. Michael's problem was with the temperature control, and the cable for that is accessed from the passenger side. Since my problem is with the function lever (vent, heater, defrost, etc.), I decided to investigate on the driver's side. And sure enough, under the dash behind the console, I found a series of cables connected together and moved by these metal pieces (I forget what they're called, but they're sort of like pulleys). I saw them move when I slid the function lever. Unfortunately, I could only access a few of the "joints" to lube, and that did not fix the problem.
So what's next? Do I have to somehow get to the other joints that I did not lube, or will I have to remove the middle of the dash to get behind the control panel to look at the lever end of the cable?
And just my luck, I opened the hood to access the cable that attaches to the heater valve (as per Curly's suggestion), but now I can't get the hood to close tight. No lube of any type or amount has been able to get it shut tight (the release cable still works fine).
I'm not a real DIY type like some here are, and with work and weather factors (no garage where I live), I may have to get my mechanic to look at these problems.