Does it look stock or does it look like a diy'er (do it yourself). My car or any mustang I have seen has anything like that. Sometimes it the person had a amp, they put a button to turn it off and on. But if it is not a toggle switch just a push button. Possibly the person had nitrous on the car. More than likely it would be to purge it, because I don't know how you would keep your finger on it and drive at the same time =) if you know what I mean.