If it never had them then you would have to get them turned on by a dealer even if you got a new bumper with them in. The fact you have the button would seem to suggest you had them at some point because I've only got one button. The simple answer is just look where you're going like I do. Even the back ones aren't needed, they are just a convenience and most of the time I turn them off because they drive you mad with a trailer on.