Frankly it makes no difference. If you remove the negative wire from the battery everything will stop, if you remove the positive, the exact same thing will happen. There are a few electronic parts that can tell the difference in the direction of flow but most in a car, can not.
Being negative or positive is only meaningful in relation to a circuit. Any break in a circuit and nothing happens. For example if you remove either prong in an electrical cord and plug it in, it can not function. However keep in mind that there may be a back door.