Hi,
You can add speakers, but when you change the number of speakers, you change the ohm draw on the radio which often times damage the radio's amp.
As far as sub, you really need to have a designated amp. Again, it will over load the radio.
I understand why you want to do this. However, most car radios are designed for up to 4 speakers drawing 4 ohms each (20 watts per channel). Adding more is likely to cause damage. I would speak to a stereo specialist about installing a head unit and amp to add more.
I hope this helps.
Let me know,
Joe
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Friday, May 1st, 2020 AT 9:05 PM