99 Monte AC/heater problem

My 99 Monte Carlo LS has a problem with the ac and the heater being stuck in the defrost mode. The heater and the ac work fine but it won't switch to anything else. I heard this could be a vacuum prob. I found a hose that runs from the top of the upper intake manifold. It goes to a T then one end to the firewall and the broken end is by the battery. I don't know were the broken end goes!!! Is this the problem or something else? Any help would be great! Its the 3.1 (3100 sfi v6)

Look under the battery area for a black canister shaped object. You may only see it from underneath.

The other end is in front of wheel well- behind splash guard- and terminates at plastic reservoir mounted there.....connect it-- battery gases have corroded that plastic line

There was not anything there but thank anyway aleekat.

I think I got it brcidd thank you. Not sure what purpose it serves though. Any help with my other problem would be great. Can't figure out why the ac/heater won't switch to the vent.

That's called a vacuum tank....stores vacuum via a check valve to provide vac to your HVAC, when the vehicle accelerates(this causes loss of vac due to the throttle plate wide open).....

Next move is to check if you have vac going to the actuators....