Check the fan motor first. If you can pull the power stage, you can check for any buildup of "stuff" on the fins and clean it. Really no way to functionally check the power stage, except to eliminate all other causes.

Trying to access all of this through the glovebox is like working through a keyhole. To really get at it, you will have to remove the knee bolster, as shown on the YouTube video. Not hard, but be gentle removing the nuts from the threaded studs. Easy to damage the threads.

There are several videos, but this one is the clearest to me:
http://www.youtube.com/watch?v=frZl49p2quk

When my system began acting up, I spent a few hours on the Brickboard and elsewhere getting smart. One thing I read is that the life expectancy of power stages is about 10 years ... and you are a bit beyond that. Sooner or later, you will have to replace it.

Of course, the problem could be both the fan AND the power stage. Let's hope not.

About defrost: Moving to that setting will turn on the A/C and place the fan on high, regardless of any other ECC settings.