what does this part do?

[vegas80step]

the ac air vent

I'm working on putting my interior and adding some speakers. I'm trying to figure out what to do with my kick panels.

What does the air vent up under my dash on the passengers side do?

I have factory AC.

I don't think my old vent door was hooked up to anything. I was having trouble with engine smell in my cab when running the AC (if that is at all connected)

looking in the hole with the trim/kick panel removed, is that a filter back there? some kind of fan?

sorry if this is a retarded question but I got no idea what that hole does and what happens if I cover it up??

thanks