A/C blows out defrost and vice-versa
 

I have a 2002 Chrysler Sebring LX Sedan.

Typically the A/C blows out the defrost vents, and when I turn it to defrost, it usually blows out the A/C vents, and not the defrost. When the A/C is supposed to be on, it usually works, but not always, and sometimes it comes on when it's not supposed to. I live in Minnesota, so this winter is going to be bad if I can't figure it out.

It doesn't seem to have anything to do with whether or not the car is idling. What could the problem be? Thanks.

mode motor or a bcm issue Id say

Another possibility is the control head, which seems to be notorious for causing problems. The other possibilities mentioned by Tech are probably a little more likely, but the control head is a lot cheaper and easier to replace, so it might be worth a shot.
The sedan and convertible from 2001 through 2006 and the Dodge Stratus sedan from those same years all used the same control head so there are many of them around in junkyards, on eBay and in dealerships. Replacement is a very simple matter, an easy DIY project.
On the other hand, finding and replacing the BCM is a lot more complicated, and replacing the mode door motor involves pulling out the whole dash. I'd try the easy and cheap fix first. Can't hurt, might work, and even if not, you're not out much time or money.

Had a couple of other thoughts about things you can try. There's a "calibration" routine you can do that could straighten out that mode door motor which seems to be operating but not correctly. Try this:
With the engine running and the car in Park, set the heater blower fan speed to 1, 2, 3 or 4. (Doesn't matter which, just not 0.) Turn the temperature dial all the way to the cold side (left). Turn the mode dial all the way to the defrost position (right). Press and hold the rear window defrost button until you hear a chime. That should put you in the calibration mode. The system will go through some gyrations on its own for about a minute or so, then, when it's done, it will display a code or codes in the space there the odometer usually reads. A "0" is good.
Give it a try, it's free and easy to do and might just work.
If your system won't go into the calibration mode with that input, try disconnecting the battery (there's a terminal for that purpose on the left spring tower under the hood) for a minute and see if that does it.

Thank you so much. This has been very helpful.

I ran it through the calibration process today, and it helped. I didn't have much driving to do, so I'll see how it works over the next few days, but so far the fan controls have responded the way they should.

However, I'm not sure about the AC. It was kind of chilly today, so I'm not completely sure, but the AC didn't seem to work when I tried it. During most of the calibration process the reading flashed "AC 00," but towards the end it turned to "AC 45." I haven't been able to figure out what that means.