Dunno....I've never seen a set of 2012 Focus seats. A picture would sure help, but I'm sure if it'll fit in a Focus, It'll fit in a Mustang. You'll have to modify the bracketry for certain though.
Sitting an a 2012 focus now.... But that's not going to help. I'd say measure. And since its snowing an cold here, and the focus isn't mine I can't measure now. I'll check back tomorrow and help if you haven't gotten answers.