My son has a cooler box in his car.
For reasons too stupid to explain he managed to connected it reverse polarity and it stopped working.
To everyone's surprise there was no release of "magic smoke" in the process.
There is a PCB with a relay and some components.
The simple answer - get a new one.
The "Annoy Dad" answer - ask me to fix it. Well I did some physics and electronics, but I know a man that knows way more than I (Kev) and I thank him for his insight.
He worked out the schematic, and suggested we have fried one of the transistors.
Well, that is easy. Even a min quantity from CPC is a small price to get more.
End result - bingo - it works.
Saves some shit going to land fill.