one of the factory auxiliary battery setups runs a cable straight from the battery to the starter.
Actually, MORE than just one of the factory auxiliary battery setups runs a cable straight from the auxiliary battery to the starter.
.
.
.
And in every one of those set ups, that OEM cable has a fusible link! It is not shown in the parts explosion diagram, because the fusible link is incorporated as part of the cable assembly. You can't get the factory cable without a fusible link.
That's why I asked about your 4GA cable, because I was SHOCKED (ha ha) to see a factory wire that fat within 18" of a power generator or battery without a fusible link. You made me know the world was right again when you said that it had one, and you had cut it off. What you do with your truck is your business, but it would have been NHTSA recall city if GM actually sold vehicles like that.
Back to electric fans... I have twin electric fans on one of my trucks. These relatively small twin fans are shrouded directly to the oil to air fin and plate transmission cooler, and are in addition to, not in lieu of, the engine driven clutch fan. Different brand truck, so the transmission cooler isn't where it is in a GMT800. But cooler location isn't the point...
The point is that I ran my electric fans for 10 minutes last night, with the engine just idling, nothing more. The truck hadn't been driven in a couple of weeks, so I was doing a periodic start, run, and exercise all systems. After I shut the truck off, and just before I shut the hood, for some reason I decided to feel the external fuse holder for the fans. It was very very very warm through the rubber cover.
I opened the cover, and sticky black goo combined with crispy res-olidified plastic greeted me. I was looking at what was left of the 10 amp fuse, which had completely melted away... WITHOUT BREAKING CONTACT! The fans could still run, and the metal skeleton of the fuse, looking like a miniaturization of the aftermath photos of the burnt and twisted steel skeletal remains of WTC, was still marginally in tact (until I touched it with insulated pliers... then it crumbled apart).
It was just blind luck that I caught it. My expectation was that a fuse should separate electrically before melting inseparably into its holder, but that didn't happen here.
Inexplicable electrical faults can and do happen. How much factory engineered protection you are comfortable rolling the dice without... is obviously your call. But that fuse melting was certainly a wake up call for me.