if you switch the positive and negative wires in a car stereo a couple things can happen.
the stereo simply will not work because all of the wires are not hooked up the same way.
if you are inserting an amplifier,
switching the wires could cause the amp to short out;
this can sometimes cause damage to the amp.
most of the time if all of the wires are hooked up the same way,
everything will work.
so if you switch positive with negative on one speaker,
you must do the same with all other connections.