2000 audio upgrade

[Mangosmoothie]

I've never messed with car audio. Decided to try revamping my sound system. I believe I have the m490. There's an amp in the frunk, rear speakers, door speakers, dash speakers. No Bose. I'm having an issue with overall volume, the rear speakers aren't working at all, and there's distortion at max volume. Car already has a Kenwood head unit. No idea what horrors await behind it with wiring.

Parts ordered so far

Power acoustik single din car play/Android Auto https://www.walmart.com/ip/5055269224?sid=d0fa6bf7-d565-450f-89e7-f8cf7e102cff

Kicker ksc40 to replace dash speakers (crutchfield inxluses a pnp harness it looks like) https://www.crutchfield.com/p_20651KSC40/Kicker-KSC40.html?tp=105

Metra 70-1787 harness (also comes with crutchfield diagram and support which I'll need since I don't know what I'm doing)

From what I gathered searching here, the rear speakers are in series and powered off the head unit not the amp. My guess is someone didn't wire them when they installed the Kenwood head unit. But I suppose it's equally possible a driver is bad so the series isn't working. I'm hoping the crutchfield documents help me isolate the issue.

Suggestions for door speakers are welcome too. I know the amp crossover only sends lows to them. Most things I'm seeing are 4 ohm so I'm worried they won't be loud enough.

The rear speakers I'm also not sure about. Are they 2 ohm wired in series stock?

[maxbottomtime]

Are you running stock amp? the crossover makes it a pain. Honestly, I regret messing with speakers on my car - the surrounds were in better shape than i thought, and honestly sounded good enough. Swapping to the kickers will make them quieter (because they are lower db sensitivity than the factory speakers). If you are going to replace speakers, might as well get a 6 channel amp. I put a 4 channel amp in up front, and the rear speakers running off the head unit are severely underdriven. debating a 6 channel amp for a future upgrade.