I put one in a buddy's MKS about 6 months back. It wasn't part of the recall, so he had to buy the part from Ford and give it to me to install. Not sure about the Flex, but it's a pretty simple job for the MKS. Just remove the rear seat (it's right behind the rear seat on the passenger side), unplug, remove a couple screws, and plug in a new one. No need to flash the ECU when done. Once the new ones in, problem's fixed.
Took me about an hour and most of that was figuring out how to remove the seat. I could probably do it again in a half hour. Not really a big deal.