Russlk said:
I don't know of any vehicle with positive ground in the past 20 years at least. All cars built in the US have the tip positive and the outside negative (ground).
I don't know of any, at all recent, positive earth vehicles either, but we're not talking about the earth polarity of the car - only of the lighter socket, and the outer ISN'T grounded to the chassis, only connected via a wire, so it can be easily connected either way.
We sell portable TV's to people to use in cars or caravans, and we get quite large numbers returned as 'not working', and they have been connected the wrong way round. At this point we ask for them to bring the car (or caravan) in and we test it, in almost all cases the centre pin is negative!.
Admittedly this is a biased sample, because the ones that are wired positive centre don't come back, but portable TV's is only a VERY small part of our business - and I see no reason to assume it's not a representative sample?.
Saying that "All cars built in the US have the tip positive" is rather a sweeping statement!, have YOU personally checked all of them?.
So, I'll repeat again! - DON'T BLINDLY ASSUME THE CENTRE IS POSITIVE, CHECK WITH A METER!.