I was wondering if when I needed to run a cable from the negative terminal of my battery when running the positive inside my car for powering a CB radio. I know this seems like a dumb question but I have not been able to find a definite answer online.
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.