[euromodder]

The negative pole on the battery is the mass or "ground" side in conventional parlance, right ?

The radiator is bolted to the car engine/frame/chassis, so it's on the "ground" side of things electrical.
The coolant, being mainly water + dissolved acids and bases, is conductive, so once again it's on the same electrical "ground" side

In order to actually do something, you need a potential difference - i.e. voltage so you can get flux - but when everything is connected to everything else, the potential is the same all over.

So what could it do ?

One thing it could have done is sort a grounding issue somewhere ...