Aftermarket Stereo system

[Lioneljeff33]

Hello all, I have a 2023 subaru forester sport. I am looking to add aftermarket subwoofers and amplifier. Can this be done on my car? I have Harmon system in my car now with small sub in the cargo area side panel. I want to add a 1,000 watt amplifier with (3) 12 inch subwoofers in the cargo area. How do I do this? Do I need a new head unit? Any help will do. Thanks.