Multimeter "false" measurement in car
If I take the negative terminal and put it on the car body the reading is now 0.

Then very likely your meter didn't make a proper contact to the chassis (there might have been paint/oil/grease/dirt preventing a proper connection).

or

There is something wrong with the battery - (minus) to car chassis connection.

Normally cars have the battery - (minus) connected directly to car chassis.

So measuring the voltage across the + and - battery terminals should give about 12 V.

Measuring between the + battery terminal and car chassis should also show about 12 V.

Car batteries can deliver extremely high currents which can evaporate (after melting and catching fire) even a modestly thick wire when connected across the battery terminals. So normally you should avoid messing with car batteries if you're unsure what you're doing.