12 volt wiring for Dometic cooler in weekender

[WVVWFERG]

I recently purchased a chest-type Dometic 'fridge (like the ones that were standard in 85-86 Wolfsburg 'weekenders") to replace the one missing in my 86. It is designed to work on either 110 or 12 volt and works well on 110. I am missing the connector for the "cigar lighter", there are two wires (one white, one black) both ending in male fittings. I, of course, want to replace the missing the 12 volt plug-in. I would think that black would be the ground wire; can anyone confirm this? I appreciate your assistanc, and am really kind of embaresed to ask such a simple question. Kim