I feel this might be an important question. "You should know" and by that I'm not really saying "you should already know" I am saying you should learn and remember this:
There are VERY VERY FEW vehicles that are 12 v and which are positive ground. There might be some oddball import that you are likely never to encounter, and there used to be a FEW big heavy trucks. I'm not sure any of these are produced nowadays Some old Autocar and Mac trucks were POS ground and 12V and "I think" White trucks
EDIT: Just looked up, some Brit cars, Jag and MG were pos ground thorugh 65
BUT NO CAR or light truck THAT IS 12V that you are likely to encounter will be positive ground
Back in the "6 volt days" things were different. There was a mix of pos and neg ground and you just had to be careful. "Back then" there were few things that would get burned up by reversing
