ALWAYS disconnect the NEGATIVE! The negative is the ground. Without the ground, the circuit cannot be complete. If you take out the positive, it can ground on almost anything metal in your car and fry whatever its hooked up to. Just like when you install a new battery in your car... Disconnect the negative ground, disconnect the postive, put the new battery in, connect the positive, connect the ground... that way you never have a chance of shorting the circuit by accidentally grounding the positive wire... Just my .02.
~mike