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

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 want to 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 going to change state of equipment by connecting opto-coupler/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 CAN bus. But not all eq. are connected to CAN bus and its a little bit risky but Im working on it as well. I`m looking for best low cost solution. I have found it on ebay . Will it work like I want? Offer DIY solutions as well.