I have one of the oldest systems in my C10, it might even be a Gen I. I have the factory air controller and mine uses cables on the sliders and the fan speed switch (the up and down one) works as it should, so my guess is that you have the slightly newer one that Cebra mentions above that uses plastic electric sliders on the the sliders instead of the cables.
I have the plastic slider system on my mustang. They look like the pic below (look under your dash, this should be obvious):
They're finicky. As Cebra mentioned, there is a "set up" procedure. You need to find the gray jumper wire and ground it out.
This is the procedure from Vintage Air. You need to follow it exactly:
http://www.vintageair.com/instructions_pdf/909950.pdf
My Mustang is still finicky at times. Sometimes it won't fully shut off, it will just run the fan at a super slow speed. Sometimes turning it all the way on high then back to off will shut it off, sometimes turning the car off and a restart shuts it off, and sometimes it works exactly as it should (this happens most of the time). The last time I "recalibrated" it, I didn't quite go all the way to the left (off). I kept it a hair off to make sure it turns off.