99 Mustang - 3.8 - a/c cutting out on grades occasionally

My a/c cut off while going up a grade, and went to defrost. Bought Motorcraft part YG360, which I guess is a vacuum cut off - but can't see where it is to replace it. I thought it would be near the firewall - Chilton's doesn't seem to show it, nor the vacuum diagram under the hood - was looking for youtube video's on it.

I understand it is either this part normally, or a part under the dash that controls the blower doors -