"Not to bring up an old post, but I am having the same problem."
Defrost only? Then you have a vacuum problem. As posted earlier, this may be the solution.
http://www.contour.org/FAQ/FAQ.php?s=probs&displayid=26
"But I just got my car back from the dealer and they replaced the upper and lower intake manifold gaskets. Could they not have hooked something up right putting it all back together? It was working before they fixed it."
May be coincidental or they may have knocked a vacuum line loose, came off or broken.
"And if it was something that they did, would they have to replace the gaskets again?"
Probably not.
You may want to see if there are any lines coming out from the intake manifolds which may be loose or not attached.
http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=v6maint&Number=931756&Forum=v6maint&Words=UIM%20aide&Match=And&Searchpage=1&Limit=25&Old=allposts&Main=931756&Search=true#Post931756
In any case, you should sent it back in to have them fix it.
