VWs have been negative ground forever. That car had few electronics to fry, so having the battery connected backwards shouldn't have ruined anything but the radio. Does the generator seem to be charging? If it is, it's polarity has gotten reversed and you may have to repolarize it.
Polarizing Your Generator
I can't understand how you would get -12 volts anywhere except by connecting the + (red) probe of the meter to something that is connected to the - post of the battery, and connecting the - probe to something connected to the + battery terminal.