Couple audio issues I'd like some assistance with

[cinosa]

So I had some stereo gear installed in my car 2 months back. Deck, 4 speakers, amp to power them. I have alternator whine, some distortion and when I put the deck into AUX, I have a LOT of distortion. So, the major issue that I have with this is that, while the amp is a 4ch, it SEEMS to be only operating on 2. I have 2 5 1/4in coax speakers in the doors and 2 6x9's in the stock location, each speaker wired to it's own channel on the amp. If I disconnect the RCA's for side B on the amp, I hear no change in output from my speakers. If I disconnect the L channel RCA on the A side of the amp, I lose both the 5 1/4 and the 6x9 on that side, same thing with the R channel on the A side, I lose both front and rear speaker on that side. When looking at the amp, I see all 4 speakers wired up, 4 for side A, 4 on side B (2 wires per speaker). The amp has an A/AB selector (i'm guessing in case you wanted to bridge side A) and when moving that, it also makes no difference in output. I should also note, when I switch to AUX and plug my iPhone in, the sound is GARBAGE, it's so bad I don't even connect my phone anymore and just listen to the radio.

Prior to me getting an AUX cable, I didn't realize how BAD this sounded, but I can definitely tell how bad the staging is. The problem is, I can't figure out where the problem is: wiring, deck or amp. What kind of things can I do to narrow this down? How can I cure the alternator whine and the distortion I'm getting?

Thanks in advance.

[SaukRapids]

For the amp, I'd find a junky speaker to test the suspected bad channel. Just use a short length of wire to eliminate any issue with in-car wiring.

My aux cable developed ground noise any time I had the HVAC on. I tried a ground noise isolator for like $10 off Amazon. That worked for a while, but it got worse again. I redid the splice properly and removed twist n tape. Instead searching for a new ground, I tried moving the cable away from the back of the climate control just as much as I could. GF says there's still a tiny bit of noise without the isolator, so she just keeps it on going to work. I tried, but I can't hear anything...

[cinosa]

For the amp, I'd find a junky speaker to test the suspected bad channel. Just use a short length of wire to eliminate any issue with in-car wiring.

None of the wiring leading from the deck to the amp is factory, all new speaker wiring + RCA's, but I'll give that a shot anyway, I have nothing to lose except some time. Thanks!

[SaukRapids]

all new speaker wiring + RCA's

Yep. Occam's Razor in troubleshooting, friend. Eliminate the wire.

I think it could be a bad channel on the amp. Outputs go bad on head units too, though.