Footwell HVAC selection not working
 

I have a 97 Ti and the entire HVAC system works, except that when I select the "footwell" option on the knob, no air comes out into the footwells.
The dash vents and defroster vents all work fine and blow hot and cold air without issue, and I can select between them.

Where should I look first? Would this be an issue with the knob? Is the system vacuum actuated, mechanical, or electric? Is this a common issue? I am unable to find any threads relating to this problem.

It's mechanical, cable actuated. I just looked, as I have my radio out. There are two cables that go to the selector switch. Actually, one is a woven cable, the other is solid like a lawn mower throttle. I'm guessing either a detached cable or broken plastic part.

Thanks paul.
I'll pull my radio out later and take a look. If anything is broken i'll probably find a way to just force the vents open as I always keep the footwell vent on anyway.