sorry if this is a little long but I really need help and don’t know what to do

About a week ago I bought an automatic 2003 g35 coupe. It drove great on the test drive, great for a couple days after I bought it. Something that will be brought up later is that it doesn’t have an aux port so the guy somehow hardwired in one of those old iPhone 4 chargers, then later added an adapter to fit the new chargers. It could charge your phone and play music through it. To use the wire you have to flip a kill switch and tune the radio to a certain frequency.
The first time I drove at night, there was an issue. While I was driving down the street the cars entire electrical system turned off then turned back on. All the lights and everything, but the engine stayed running. After it turned on nothing changed and it drove good, except the drivers side window went up a little slower than passenger, and the electric door lock didn’t work, but I could still manually lock it with the switch and with the keyhole. Also I noticed the music would get super quiet for long periods of time when I was using the phone cord. I turned it to the regular radio, turned off the switch. and it sounded fine. later on, while I was stopped at a red light it did it again, but this time the whole car including the engine turned off. I had to put it in park, restart the car. It started up and I drove off and made it home fine. The next day I drove around fine during the day, it ran great. But when I drove at night, it did it again. But this time the battery was completely drained and I needed a jump start. It drove home fine after the jumpstart.
So I took it to autozone and had them check for codes, there was none. Then I had them check the battery and the alternator with the hand held device. He said the battery was good but the alternator was bad. The device said it failed when testing for “recharge current”. I took it home and took out the alternator. Before buying a new one I had Orielys bench test the old one. The bench test said the alternator worked good. I took the old alternator home because I didn’t want to spend money if that one worked. I put back in the old alternator. At the same time I also used a multimeter to double check the battery and it said it was good. At this point I noticed that the terminals, and especially the positive one were pretty eroded and dirty. I deep cleaned those, put new battery gel on and made sure everything was properly connected. I went on a ~20 min test drive with all the electric stuff turned all the way up ( heater and fans on max, music all the way up, heated seats, and even had the brights on when I could. I was using the phone cord and the music sounded good). I thought maybe cleaning the connections fixed it.
The next time I drove at night I drove for a really long time. it was fine except the music sounded bad when using the phone cord(quiet-ish, and really bad quality). The regular radio sounded good. Also the window and lock still acted weird. But other than that there was no problems.
the next time after that( today) I drove it at night, it ran fine, no issues except, again, the music was bad when using the phone cord. This time it was just super quiet, max volume sounded like 1/3 volume.

After those two times driving at night (combined ~3 hours of driving) I would now think that taking off the alternator and cleaning the battery terminals fixed it if not for the music situation, and the slow window and lock button not working. Other than that The car runs how it is supposed to and you would never know it was acting strange.

here are my questions and thoughts:
-does the music situation mean anything? It is a weird rig and maybe that’s just how it is but i don’t know
-the lock not working and window being slow could also be signs of a failing alternator
​​​​​​-the alternator (specifically “recharge current”) failed while in the car but the bench test said the alternator was good. I still think the alternator might be bad. I’m going to have them do that same test while it is on the car again after I did and see if it’s any different.
- maybe it’s the voltage regulator, I heard that going bad can do the same thing as an alternator failing.
- maybe a ground somewhere is bad? I don’t know how to fix that so hopefully not

any input is appreciated. Please help me I don’t know what it could be or even if the alternator is actually bad. If it’s not I really don’t know what could be wrong.

 

Sounds like it was more of a battery terminal issue and less of an alternator issue.

Stereo sounds like it's own monster.

If the window motors are sluggish that's a sign that the motors are failing.

So the door lock actuators don't work AT ALL from either the door switch or the fob button? Neither side works?

 

I forgot to update, but I took the battery to get tested and they said it was bad. Weird because before they said it was good but the alternator was bad. But they said the battery could charge fully, but just wasn’t putting out enough power. I replaced it and haven’t had any major issues.

yeah I am planning to just replace the whole stereo with a double din touchscreen one.

the passenger door always works with the key fob button, the inside button, and the silver mechanical switch. The driver side door locks when I use the silver switch and the keyhole. But it does not work when I use the electronic button inside or the key fob.

one more thing, I just found out the previous owner added the unorthodox underdrive pulley set ( Includes Crank Pulley, Alternator Pulley, Water Pump Pulley) and that may cause issues with charging. Could I put a stock pulley back in the alternator and leave the other ones and would that help?

 

I strongly ditching the crank pulley and going back to stock, especially since there is absolutely NO harmonic damping on that pulley and your crank/rod bearings are taking a beating with it.

I believe it's just a solid aluminum lightweight pulley but it should still be the same diameter as stock, there are issues with the variable geometry AC compressor if you try to use an actual underdrive pulley.

As long as it is a stock diameter pulley then you can just keep the lightweight alternator pulley but switch back to a stock harmonic damper crank pulley.