marine1964,
it would be better for you to start a new thread of your own when you have a different question. However I will give an answer here. There are small power supplies that you can buy that will provide 12 volts Dc from 6 V Dc. However, if your GMC has a positive ground setup, finding a power supply to do the job might be difficult. In that case I would suggest converting the truck to a negative ground system.
