Tapping Into Car Battery
In quite likely 100% of all vehicles on the road, the negative side of the battery is firmly connected to the chassis/frame/body of the vehicle.  It is very common to use the metal body of the vehicle as a convenient path to "ground".  You are quite safe assuming that the metal body is ground by definition.

Ref: http://www.w8ji.com/negative_lead_to_battery.htm