92 Pontiac Grand Prix Heater Question

I have a 92 Pontiac Grand Prix and suddenly, the 'vent' switch doesn't
shift the air flow through the vents on the face of the dash. Blower
only blows through the floor or defrost. Is there some sort of relays
that may be acting up and not allowing this to switch properly? The
heat control switch on the dash indicator lights function properly but
the actual switching of air isn't taking place. This problem seemed
to coincide with when I changed the car battery but that may just be a
coincidence.

Summer heat is such that the AC just can't get enough cool air into
the car through the floor vent so I'm anxious to get this fixed. I've also heard about vacuum line problems that can cause this but haven't uncovered any vacuum issues and my engine runs fine (vs one that has a vacuum leak

thanks very much for any pointers...