I've delt with a few water boxers Bad Ground is common, and I'd check to see what the lady at Kmart sold you. Like He said 13.5-14.4 IS acceptable voltage at the alternator. And baking soda water and a good wire brushfor those posts, that corrosion could cause you many problems like a dead batterie. As the alternator nolonger has a proper charging and Ground Connection. For a GLX with options like power sun roof it should be a Larger one but For the VW There is not alot of options for size. As VW keeps it simple. IF I remember correctly should be a 500 - or 600 that you'll get aftermarket. 540 Is what the factory had in them, I had a 96 to.o Hope that helps.