No air coming through dash vents

I own a 1994 Lumina Van. When I put the fan on for heat and want the flow of air coming through the vents on the dash, it takes about 3-4 minutes before the air flow will switch from window defrost to the dash vents. I had some leaks repairs on my transmission lines 3 days ago and the mechanic sprayed the inside of the engine with water after he was done.
The van has been outside in the cold since then. Is it possible the "trap door" that changes the air flow could have froze up?
If not, what could be the problem?

Yo, i have seen some cars that won't flow no air into the dash vents until the air is cold or hot, depending of the case, so i won't flow hot air on your face when is hot or cold air when is cold.
In the mean time, my car stop blowing air trough the dash vents at all, and i still trying to figure out how to fix it.

Yo, i have seen some cars that won't flow no air into the dash vents until the air is cold or hot, depending of the case, so i won't flow hot air on your face when is hot or cold air when is cold.
In the mean time, my car stop blowing air trough the dash vents at all, and i still trying to figure out how to fix it.
I had a similar problem on my 97 lumina found a open vacuum line, trace vacuum line from manifold (mine was small plastic (ridge type),maybe 1/16") throughout system. Check for cracks or splits. Mine was leaking at the left lower front fender where the canister is.
Let me know if this helps!

I ended up taking the van back 3 times to the repair place before they were able to fix the problem. When I had a new water pump installed last week, the mechanic put 2 vacuum hoses back on in reverse. It works fine now.
Thanks for all the replies!