alternator whine soldered rca?

INFO:
1994 Mitsubishi 3000GT SL
Phoenix Gold wiring (4ga, 8ga) to amps with a fuse holding distributor block /w 0ga from the battery, Phoenix Gold Ryvaal V8001 1600W MAX mono amp powering Elemental Designs 16OV.2 dual 4ohm sub @ 2ohm parallel load; Kicker ZX200.4 amp powering Infinity Reference 6012i's @ 4ohms. Big 3 upgrade also done using Phoenix Gold and SoundQuest wire. Only thing stock is the headunit.

Link to what I'm trying to do:
https://www.3saudiofaq.com/6pintorca/page1.html

Using the above site for directions, I soldered RCA connections onto the stock DIN plug that comes out of the back of my stock h/u to the stock amp (which is removed), and With just acc on everything sounds fine. But I get alternator whine once the engine turns.
My guess is a bad ground on the soldered RCA connections I did and/or interference from the 8ga and 4ga power wires approx 8in away from the soldered RCA cable connections. The only insulation the exposed wire has is electrical tape.
The amps are not the problem, without the RCA plugged into the amps, there's no whine. Whine only comes on when the h/u is on (aux, radio).
Anyone have any tips on insuring a good ground connection when soldering a bunch of RCA cable grounds to the one ground lead from the h/u? And any tips on what I can use besides electrical tape to help eliminate interference? Thanks.

How about all 4 shields soldered to 16ga wire to chassis; ground wire from DIN to chassis using some more soldered 16ga? I'm not sure if the ground wire from the DIN to chassis is necessary or not...

You will have to try it both ways.  If you are going to try it, simply ground a wire and touch it to what you have already.  If the noise goes away, simply solder that wire to the shields.  I am really thinking you need to remove the ground from the din plug, but try what you suggested before you remove the shields.

I think you misunderstood me. I'm removing the shields from the din plug ground wire and the shields will all go to chassis and not touch the din plug ground wire. The din plug gound wire will also go to chassis but at a different spot.

I think I had alternator whine initially and then I got engine noise later as well. Grounding the RCA shields to the chassis + shorted the DIN plug ground wire got rid of the whine; fixing the gain got rid of the engine noise. However; my sub routinely goes out and it doesn't sound right.
No more time to work on it today; but I'm gonna switch the sub's speaker wires tomorrow and hopefully that'll fix the problem, maybe adjust the gain a bit more on the sub amp. Once everything works for sure; I'll post pictures!

Thanks idiot; I have no alternator whine at all anymore! Nothing wrong with the 2-ways, but I noticed there is a hum coming from my sub. It only occurs when the sub is connected to the amp; doesn't matter if RCA's are in or not as long as the sub amp is powered on. When I hook up my sub to the other amp (220W vs the sub amp's 1600W) there is no hum....
Could it be a problem with the wiring of the subwoofer? I have a 4ohm dual voice coil sub wired in parallel using 8ga wire which is pretty damn big, maybe some + and - wires are touching each other causing a ground loop or something.
First gonna try switching grounding points for the sub amp to see if that makes any difference.

what a freaking mess.
alternator whine is back. so is interference from who knows what.
sub still hums, and doesn't always play music when the engine is running - and when it does, it's about half as loud as when the key is in acc.
def not the RCA's, plugged both sets into both amps with the engine running and only got sound from the kicker amp, not the phoenix gold.
adjusting gain doesn't get rid of the hum from the sub.
the guys at phoenix gold tech support were all baffled as am I. gonna try and see if one of my friends can come by and switch out amps for a second to see what we can learn.

Problem isolated to the Phoenix Gold amp. It sounds strange, more boomy than it should be in a sealed 1.9cf enclosure, and it also goes on/off intermittently when the engine is running. Sounds fine when the key is in acc position. PG tech support still baffled...I may have to get another amp....