What is the box under the dash. It says cavity?

Today I was cleaning the floor boards of my truck. I felt back under the dash right under the radio.
There is a box there with the word "cavity" on it. It was very cold. I think it has something to do with the A/C since it was on. But the box was cold and almost like a cooler. I'm just curious as to what it is?

I can't seem to find it on a parts diagram. I thing it must be a cover for something on the A/C
I will turn the A/C off tomorrow and see if it stays that cool.

 

Probably so. Is it broke? Then it's good. You got the new worm drive? Then it's where the interstellar mass resides in a tessellated vacuum cavity.

Hehe, poke a finger at it, if it jumps like a patient, then it's a different type of cavity.