Sometimes the thing screws appart and you can see where the wires are going. They are right the center post is Power or POS.

So you don't have a test light? You can take one wire from your plug, put it on Neg, take any 12V bulb and plant the center on the Pos terminal and touch the side with the side of the Cig plug metal part (Use a short wire if you have to) If it lights you got it right mark that wire BLACK or Ground. (Other is POS) if it does not light switch the wires and try again until you get the result.

Test lights are cheap.

Pos terminals are almost always the hardest to get at. In this case the tiny thing in the center. In a two prong plug POS would be the insulated wire and the exposed one the ground. (On live ends)

Normally when two wires are black the Pos has a white line through it, sometimes grey.

Wire it up and plug something in that won't burn up. like a LED light. That type will only light if you have POS correct.