What is the cheapest and best method to convert 12v(14)<->5V(3.3)? Need to connect aruduino to a car.

P.S. As I`ve said I need logical shifter not buck converter to connect arduino to car 12v circuit. I would like to control electrical equipment of car by arduino. For example, I wanna connect input pin to heater, when heater is on from 12v circuit I get HIGH 12v signal which processes by arduino, but like everyone knows, arduino can handle only 5/3.3v. Then arduino gonna change state of equipment by connecting optocoupler/relay/transistor to  button(of elect. equip.) and will act like physical press. 
P.P.S I know that more clever to hack and control by canbus. But not all eq. are connected to canbus and it`s a little bit risky but I`m working on it as well. 
I`m looking for best low cost solution. I have found it on [ebay][1] . Will it work like I want? Offer diy solutions as well. Thx. 


  [1]: http://www.ebay.com/itm/Level-Shifter-Logic-Level-Converter-12-Channel-5V-to-12V-or-12V-to-5V-/251604416861?hash=item3a94cab95d:g:DUYAAOSwxH1T2d~7