Hello, new lexus owner and new to the forums as well. 2004gs300

I have searched and searched but am still so confused so about how to go about this.

I want to put new door speakers in and eventually subs. I would like to keep the stock NAV unit if possible, it seems too complicated to remove it and buy the HVAC control, and i found lack of information about people doing it. I read something about using line out converters. If i replaced the door speakers, could I power them with an aftermarket amp keeping the stock head unit? Would they even sound good without buying a new amp, I know the subs would need them.

If upgrading the speakers without replacing the nav unit isn't even worth it, I wish i could find on more information on putting aftermarket stereo in the NAV models.

Any help would be appreciated