It's a big plastic cover that comes in the non A/C equpped cars. Air is going to follow the path of less resistance, so instead of going through your radiator like you want, it's going to go through the big gaping whole where A/C used to be.

Ya'know, I just looked at pics of my car, and you can't see the plate from the angle we're looking at so maybe you have it, for instance I have one, but you can't see it:


-PHiZ