I am posting this on behalf of my friend. The car in question is a 2012 Ford Fiesta Titanium TDCi.
The mist is formed on the inside of the front windshield after 10 to 20 minutes of AC usage in the car, its more prominent during the night. The windshield on the Fiesta is long and mist forms on the lower part first before spreading to the entire surface in sometime. Using wipers will result in faster mist formation and clouding of the complete windshield.
Here is a video of the same taken the other night.
The 2011 Fiesta has a large vent on the top of dashboard underneath the windshield which normally blows hot air when the ACC mode is set to demister setting. We suspect the problem is due to some fault on this vent which always blows out air no matter what the blower setting is. When the demister is switched on, the same vent blows hot air and the misting disappears in about 20 seconds, but it again starts after 15 minutes when the demister is switched off. Its impossible to drive the car around with the demister switched on all the time.
The vent as seen through the windshield

The vent as seen from inside the car
What is Ford A.S.S is doing?
Took the car to service center close to five time, twice during the regular service and three times specifically to address this problem. Once, the car was left in the service center for close to a week for inspection after a complaint was raised to Ford. Every time, they just give the car back after a wash and the problem just doesn't go away. During one such service, they even left a dent on the left rear fender but that's not for this thread.
How is the car driven now?
Earlier, a sun shade used to be kept on top of the vent under the windshield. Not at all elegant but it slows down the mist formation considerably. After Ford tried to fix the issue, the misting became more prominent and the sun shade couldn't help. So, the vent was closed with a piece of plastic and some duct tapes. It helped until Ford removed them all when the car was finally taken to them for the fifth time.
Our not so elegant solution to the problem
What is Ford A.S.S is saying?
Ford claims its due to the following reasons.
- Powerful AC

- Common with all Fiestas, they checked three and all of them had this apparently. But I wonder how others manage to drive without complaining.
- Very low temperature on ACC causing the mist, supposed to be driving around with 23 C as per Ford. Tried that too and mist formed still.
The help or the lack of it from Ford has been pathetic and no one with technical expertise or knowledge on the issue has been on the issue thus far. The lady from customer care calls back every now and then asking about the feedback on the service and its annoying. Unable to use the car at night due to this issue.
Has anyone with 2011/2014 Fiesta come across this? Is it really an issue with the blower and the vent atop the dash? Is the vent supposed to let out air only when the demister is on? Is it 'open' all the time on our Fiesta causing this issue? Any help will be much appreciated.