Ebay Digital Voltmeter- How Do They Do that

StewB wrote:

RJsfishin wrote:
The meter, was it $2.57 retail. including shipping,....how do they do that ?
http://www.ebay.com/itm/DC-0-30V-Voltmeter-LED-Panel-3-Digital-Display-Volt-Voltage-Meter-for-Motorcycle-/131095052802?pt=LH_DefaultDomain_0&hash=item1e85dfc202

I just received two of these. I got one for my RV and one for my cabin.
Not being to electronicaly inclined I have a question.

The monitor has three wires, red, black, white. Connect red to + and black to - and the display lights up. Then connect white to + and voltage is displayed.
My question is, what is the best way to wire this to a switch so the voltage is displayed with the push of a button?
Thanks for any help.