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GM has done different things in different years, different wiring options. Sometimes the auxiliary battery is isolation, sometimes it is a 24/7 parallel setup. I think the key is understanding differences of each and making the right decision for your own needs, including whether or not you wire as the factory does or not. I am not sold on an schematic or parts until I actually go and purchase them. Even though I do have a relay here that would likely suffice, in the back of my mind I am subject to ordering this part for it's heavy duty aspect and it the fact that it provides flexibility in wiring up tell-tales. It provide electrical contacts to tell you when the battery is engaged and when the battery is not engaged. I am sure this can handle the amperage of a large group of batteries connected in parallel. In my grand overkill plans, I am up to four auxiliary batteries that will likely never get installed.
(Correction, I thought this was listed at 1000amps. Maybe there is a second similar model. 500 amps still more than enough.)