+12v ground vs (- 12v) ground. what's the difference?
The short answer:
Theoretically there is no advantage between negative ground and positive ground. The main thing is that the potential difference is 12V (or 6V in old cars). The short answer is that it is convention (just like why do we use 110 V in housing circuits rather than 100V).
The long answer:
Imagine the following situation: You have 2 terminals A and B. A is at +16V and B is at +4V. The potential differnce between them is 12V. If you connect a light bulb between them it will light up and produce the light as if connected to a 12V source.
Now imagine A being at +8V and B being at -4V. The potential difference is still 12V and the bulb will still produce the same amount of light or draw the same power.
Now imagine a third situation with A being at -12V and B being at 0. The bulb will still light up with the same brightness as in the above two cases.
So there is no difference in any of the 3 circuits.
Way back when, around the 1930s or 40s (maybe even earlier) auto sytems had all types of electrical systems, negative ground, positive ground, 12V sytem, 6 V system. You can imagine the problem if you wanted a jump start--you had to know whether you had a positive ground, negative ground, 6v or 12 V--this would be utterly chaotic, not to say dangerous. Plus all the parts manufacturers (such as alternators, regulators etc.) would have to come up with different electrical designs depending on the car.
So some bright people in the automotive industry, decided to standardize on the 12 V negative ground system (probably because more manufacturers used it than any other system). And all that chaos finally ended. Today, with all the electronics in the car, reversing polarity would be very injurious to your pocketbook as it would fry many of the electrnic components. Hope this sheds some "light" on the 12V negative ground situation.