Remove passenger hush panel on bottom of dash under glove box, ahould be 3 7mm holding it on. look up under dash by the kick panel, it's located right bellow the glove box fuse panel. It's shaped like a harmonica, with a wiring connector and a vacuum hose connector. You will probably hear a slight vacuum leak coming from it.

BTW a standard hvac doesn't tell us what you have. Digital, manual, dual heat etc might help.