A/C-Heat vent system stuck...

Hi all. New here and looking for a bit of information.
I have a '98 CV and the vent system is stuck in the "heat" position. My a/c compressor is working absolutely fine but the vent won't switch over when I select a/c. I just keep getting hot air (and it's starting to get hot in NC. Does anyone have a diagram or tell me where I need to poke around and see if I can get whatever might be "stuck" to work again?
I've inquired about having it worked on but from what I've been told, the labor costs involved will be pricey.
Thanks in advance for any help and info.

Do you have access to a thermo heat valve that is on some 'ford' vehicles .... near the heater coil on the engine compartment side...? My 1991 Lincoln TC w/4.6 L has this valve...I know on other engine sizes (5.0 L) and newer model years...they started placing this thermo heat valve in different locations...later to change the whole concept...digital controls/valves from sensors...

Do you have a service repair manual for your 98 CV......that would be a start...or check w/your local library to see if the subscribe to a 'repair database'...that could give you some great reading materials to create a master plan to identify the location, parts & your DIY labor...