Your reply is a more emotional and not founded in facts. I’ll reply with clarifications in chronological order, as best I can.
You ask, “Then why do it?”
What I didn't mention is that I’m completing a complete wooden (MDF) infrastructure build on the rear of the car over the entire rear area behind the seats, including the area where the original spare tire and Bose sub was. I’m doing risers and shelves and compartmentalizing for the multiple amps, 2nd battery and caps and then carpeting everything to look nice and match the interior. The cables are run from the engine compartment, under the outside of the cars underbelly and back up through the sheet metal (using heavy duty nylon grommets) and then through the false bottom I made. The build includes a sub enclosure across the width of the rear compartment, fitted behind the fastback angled seats which house two Focal 33v2 polyglass subs in a 3.44 cubic foot, sealed and stuffed enclosure.. This “built” section pretty much contains everything in the system. Doing this covers up a lot of the available "short" grounding points and I didn’t feel like cutting a couple of big access points, to the seat bolt grounding points, or drilling for new ones (consolidating, grinding surfaces or layering terminal rings) given that the car is a unibody configuration. Having closed off access to a lot of metal that I might use, or just prefer not to use, my choice. I was able to source the 2/0, high strand OFC cable , 25’ for $50.00. It wasn’t that much of stretch when already planning to pay a shop to run, initially, only the power run from front to back to add the 2nd, larger gauge cable installation to the initial request; two birds with on stone. Is this type of installation effort for everyone? Certainly not. It is just as affective, yes (when done right and using extra large ground return cable) and to suggest it isn’t is also a fallacy. Not only have I consulted other Electrical Engineers, including both my brothers, but just for shitz and giggles, 2 (very reputable) car stereo installation shops (one of which helped me on another of my previous cars, and my sons car, doing the exact same thing). On simpler setups, I can (and do) see the merits of doing ones grounds the traditional, practical way. It’s much easier to insert cables into blocks and use an allen wrench to secure them as opposed to drilling, grinding and grounding 6 grounds of different gauge values to several points on a unibody chassis that may not facilitate the grounding well enough to begin with. This way you never have to worry about ground loops, you have one single ground point for everything, which electrically speaking, is Ideal in the audio world.
You recap and ask for clarrifcation “"Doing the big 3 in a big way"? WTF does that mean?
It is simple really; I used LARGE (bigly) gauge wire to augment the existing wires in a normal big 3 upgrade. Excuse me for using adjectives to describe the magnitude of the 2/0awg wire used.
You state “You talked a little about unibody, but didn't bother to write a complete thought”
This is really a no brainer and if you Google it, will find more info on it, Google something more specific to this conversation, and you see that what I have stated is factually correct. (not without exceptions, but correct) Here is a link too many replies that again, don’t necessarily like this option, but in the same breath, mentions that the only reason they don’t see any benefit is that they don’t see it as better, not that it doesn’t work – opinion, not fact.
https://forum.expeditionportal.com/threads/ground-for-dual-battery.135315/
You state “ You want to tell me I'm wrong, but won't say why”
Well maybe I could have worded it differently. Like - your method of connecting everything, from what I glean from your posts, is fine, will work, and is the most common way to do it. Your assertions that what I am stating here is wrong, that’s incorrect, and by extension, you are incorrect on this point, so only half wrong, my bad.
You state, I said: “- "Youtube it". That's what I call a "******* get out of jail free card. Go ahead and post those videos and see what this site says. I promise they won't side with either person, they will go with what is right. "
This isn’t a contest. I learn from some and provide valid, helpful information to others (as assume you do). My statements and experiences are based on 42+ years in the electrical, car audio/home theater, computer board surface manufacturing and telecommunications fields. I know,
unequivocally, that what I’ve said in this post is factually correct. When I initially started doing this type of install, I found nay-sayers such as you and the like to be fairly prevalent so I did my homework to absolute certainty when possible or I won’t post about it, for the most part.. I have engaged other EE’s, namely by two older brothers who reinforced the accuracy of my knowledge on this subject. I’ve provided enough info on this response (including several supporting links) and I do not feel the need to supplement the homework you clearly need to do with anything additional - on my time
You took offense to my ‘People don't put fuses/breakers on their grounds” and remarked: “NOT BECAUSE THEY ARE LAZY, like you try to suggest, but because it's wrong.”
How you interpret what I say is due to the way I say it, but to a larger degree, based on your perception and of your own volition. I never said or implied that people who don’t do this are lazy, again, it’s an option that I choose to incorporate in this type of build, nothing less, nothing more.
Additionally, you asked: Give ONE GOOD REASON why you would want your circuit's ground to no longer be there?
WTF, I NEVER said anything of the sort – the whole da*m discussion is about an “alternate” grounding method, not eliminating it! Again, your assertion or misinterpretation of where that ground point terminated led to that conclusion, and not what I stated.
You ask “How do "fuses/breakers protect from feedback?"
Bad choice of words as I was not talking about musical (signal) feedback but was referring to current feedback loosed to a wire from a short to chassis or an electrical failure or disruption that can occur then travels thru the power or ground cables to affect the car or the audio system, sorry, should have been more detailed. It’s not likely to happen if you use proper grommets and insulated wires, but unexpected things happen and a wire on fire is a car on fire when you’re talking about this type of large gauge installs. Better you pop a breaker or fuse and prevent the situation where you come out to your car and find a large ashtray instead. Breakers or fuses at both ends take the likelihood that this occurs in either direction, away.
Last but certainly not least, you said, “No it's not a matter of using "existing connections", your battery grounds are existing connections, RIGHT?”
In this and any situation, the battery terminals facilitate ground “pass through” to the chassis or the engine. The battery is NOT the termination point. Here is an article that explains that.
https://enginediary.com/ground-battery-to-engine-or-frame/
Before you jump all over the validity of my install method(s), go find, for your own peace of mind, information that helps you understand that this is indeed true, not more information that simply reinforces what you already know about the most common way to install and upgrade a cars electrical system for car audio applications. Again, as I have already stated, it’s not for everyone but for me, in this type of installation, I prefer it.
Remember, uninformed or marginally based “opinions” are like azzholes, everyone’s got one.