If it's got an alternator, the chances of it being positive earth are very small indeed. While there was a positive earth alternator kit offered as an official upgrade, they are very, very scarce. Anything likely to have been fitted in more recent times will have made a negative earth conversion essential. Likewise, if a modern stereo is fitted, it will be have to be insulated from the body (including the aerial) to work with a positive earth electrical system. With a dynamo equipped vehicle there's little consequence to swapping polarities. The heater motor is the only item which is dependant on polarity. Being a centrifugal blower, it will still flow inthe same direction, but the vanes are optimised to blow mre efficiently in one direction than the other, so if spinning backwards, airflow is reduced, Cheers, Bill.