How to connect 12v wires from car to head unit?

Ive tested all the wires coming out of the car, with a voltage screwdriver, and i cant find any of the 12v ones, i did a wire right from the car battery to the headunit and the had unit worked, but it never turns off, but ow come it wont work through any of the other wires from where im ment to conect them, its worked bfore but i disconnected th whole stereo, what am i doing wrong ive tried everything groundings all fine why isnt this working please help me ive made like 15 qustions about this and nothing working