Some ECOBoost fiestas have this button. If I was to fit this button would it just work or would this need some kind of Forscan activation?

 

I can't say wether you can retro fit it or not but in my experience it's not worth the effort. I can do a better job at being economical with fuel by driving with anticipation, large gaps to the traffic in front and careful use of the throttle.

 

I did a 400 mile round trip 50% with Eco on no A/C and 50% without Eco with the A/C on and it made hardly any difference.
as Blatto says its more about how you drive.

Never seen a post about anyone trying this mod, but as the eco mode affects various functions including throttle response, cruise, fan speed etc, I'm sure it will be a case of more than just adding a switch, and some reprogramming, or perhaps extra components, may be involved.  

Like other posters so far, I've had these on several cars now, including my current one and never noted any economy benefits and just found they made the car unresponsive and less enjoyable to drive. Some people seem to rate it, though - as with most things, personal opinion.

i wouldn't call it a worthwhile upgrade to be honest i sometimes use it but find mpg improvements are minor but ac comfort is reduced along with throttle response and top end performance. today i completed a 70 mile round trip and achieved 66 mpg mostly motorway with the eco button off with ac off on the way there and on on the way back