If this is stock then it is probably a 6 volt system. If you look at your battery cables one cable end will be larger than the other. The larger end cable is a positive cable. If that goes to ground then it might be a positive ground. Be very careful though when hooking up a battery, if it is hooked up wrong it will spark badly. You could always contact the plymouth club or chrysler club in your area to find out about the positive/negative ground.