Bringing this back up.
I recently had an issue with only my MAX AC going to defrost. Did some research and found out about the orings. Replaced all (4) and now it works like a champ. Just yesterday (Sunday 4,11,2010) I have a buddy who's '03 was staying at defrost all the time. Sometimes during accel or decel it would blow throught he vents, but not often. Had him bring his truck over and we changed out his o-rings. His ring had hardened up so much they would break off. No elasticity what so ever. Changed his and now working like a champ.
Ford wants upwards of $800 to replace the whole control unit. They won't "rebuild" it.
The o-rings are a #007. 9/32" OD and 5/32" ID. You can buy them just about anywhere. I bought mine from Grainger and paid $2.39 for a pack of (100), yes 100 to the pack for $2.39.
If anyone would like to try this, send me a PM for my mailing address. I'll give it to you and you can then send me a self address stamped envelope. Once I get it in, I'll put (4) in the mail back to you, free of charge.