How can I get rid of the distortion in my car speakers?

You need to examine everything present. Car stereos usually play best with 4 ohm speakers. The output of the stereo should not be more than the speakers are rated to handle. That is another unknown to us, but you can look at the speakers to see what their specs are and read the literature for the stereo. Then you need to see to it that the speakers are connected correctly. The directions should be in the literature that came with the stereo.