If you own a last generation Renault Clio, you must know that there is a hidden feature that you can unlock by itself. A button that allows you to put the car in ECO mode, lowering the power and fuel consumption on long journeys.
you can see the installation of a standard button in this video.
https://www.youtube.com/watch?v=lSSiZ8hzTz8

but the standard button is expensive and difficult to find, so you can print a housing and then connect the button you like. 3D files include a housing where you can add the right hole for your button and a pre-drilled for a standard 16mm panel button.

you need a "momentary" button that closes the contact between the two terminals only when it is pressed and not that gets stuck. because you have to press once to activate the eco mode and once to turn it off.

if you want to add a little 'style, you can choose a button with integrated LED, because the electrical connections of the hidden arrangement under the plastics includse the power to light up the button when you turn on the headlights in the night

link for an example button: http://www.ebay.it/itm/White-Angel-Eye-Metal-LED-Momentary-16mm-Black-Push-Button-Switch-12V-SPST-/

There are three electrical connections (ignore the 4th), you can see them in photographs and drawings, black is the Ground of the car, white is the signal for the button and the blue is the voltage. If by mistake you make in touch the black and blue will have to change a fuse.
you need a 150 ohm resistor because the LED (in my case) is at 12V, while withe the engine on the car battery voltage is up to 14.3V
+if you don't have the black cable, you can connect to avery metal part of the car, nut and screw for example

other info here: http://favremod.altervista.org/modalita-eco%C2%B2-montaggio-tasto/?doing_wp_cron=1485692793.4285740852355957031250

PS: the button works like a charm, no damage or risk for the car, but I do not know if it void the warranty. if you need to bring the car in an official workshop, I suggest to remove the button.