The above wiring installation is the end result of utilizing 100% GM Parts to implement an Auxiliary Battery installation per GM's RPO (regular production option) "TP2".
Current flows from the postive terminal on the Auxiliary battery to a common stud in the Megafuse block, which is shown opened in the above photograph. This common power stud is simultaneously fed by charging current flowing from the starter, by way of the Auxiliary Battery Relay and not one, but two fusible links.
The first fusible link is from the common power stud to the relay, and the second fusible link is from the relay to the starter. It is important to consider that a high current electrical short can occur on either side of that relay, so both sides are protected. Any home brew dual battery installations would be equally advised to follow the OEM pattern and consider circuit protection on both sides of any high current isolating relay.
The Megafuse to the load is stepped down to 40 amps, since the load is routed directly to Stud 1 of the UHBEC (Underhood Body Electrical Center... aka fusebox). Normally, power from the main battery is supplied to Stud 1 by way of a 40 amp J-Type clear window slo-blo box fuse installed in the UHBEC fuse panel. However, with TP2 installed, the 40 amp J-Type fuse is REMOVED from the UHBEC, leaving Stud 1 unpowered by the main battery. Then, the lead coming from the 40 amp Megafuse mounted in the separate black box depicted above is eyelet mounted to Stud 1 in the UHBEC.
Stud 1 feeds power to the Trailer Towing connector, which supplies 12 volt charging power to the emergency breakaway battery, and, potentially, power to the interior lights of a small utility trailer, if the trailer is not equipped with it's own battery. Stud 1 can also power any other loads added to the stud, provided that the total load does not exceed 40 amps.
Alternatively, one can opt to not install the eyeleted lead from the Auxiliary Battery onto Stud 1, and instead leave the 40 amp J-Type fuse in the UHBEC panel as is. (don't have both connected, otherwise the batteries will be paralleled all the time, regardless of relay position, which can drain both batteries, as well as overheat the wire between them).
If one opts not to have the Auxiliary Battery power Stud 1, then one can use the independet MegaFuse box as a power point for custom loads, like communications equipment, or a big fancy thunderous stereo. In such case, the anticipated auxiliary current needs might exceed 40 amps, and in such a case, with adequately sized load wiring, one could install a higher amperage Megafuse in the megafuse box depicted in the photograph above. In that case, the installation might mimic other GM RPO codes for the Emergency Services vehicles, such as the Police Tahoe.
However, it is important to note that in those types of installations, GM provides a separate load center panel with subfuses and relays. They also provide a very hefty rubber grommet for the firewall pass through to bring power from the Megafuse to the subpanel. One is behooved to be mindful of the safety devices GM engineers into their electrical circuits, keeping the what if's of catastrophic accidents and even minor collisions in mind.
I have not found a clear picture that makes the often shared but somewhat confusing GMPart's diagram for the factory wiring installation of RPO TP2 more vividly understood. Hence, while in picture posting mode, I decided to post an annotated color picture of a real world installation, in this case, my own, undertaken several years ago.
In my research, there was one picture post that I found most amusing, from around 10 years ago. It was of the infamous "Lonely Connector" that is found dangling unconnected on the passenger side firewall in most GMT800 truck gasoline engine compartments. The photo above is intended to clarify what that lonely connector actually does, and confirm that it is indeed for sending the activating trigger voltage to engage the factory auxiliary battery, if and when installed. Note that diesel equipped GM trucks do not have a relay, as both batteries are constantly paralleled.
So, where does the little red box (not shown) come in? You know, the red plastic box on the driver's side of the engine where the alternator and the main battery meet? The answer is: that little red box is not involved in GM's installation of the 2nd battery.
Both batteries meet at the starter. It is through the positive post on the starter motor solenoid where charging current is fed from the alternator to the starter to the fusible link in the lower right hand corner of the photo above. Then the internal coil position of the relay determines whether the charging (and depletion) connection continues on to the positive terminal of the auxiliary battery, or remains isolated.
Note that the relay trigger circuit (aka the lonely connector) still has to be completed elsewhere in the vehicle. The circuit is intact from the connector to the UHBEC, but there are no wires from the UHBEC to the Mid Body Electrical Center (MBEC). This is where the hard work comes in, if you want to EXACTLY replicate the factory wiring. I finally completed this hard work about a year ago, removing the UHBEC and the MBEC altogether, adding the wires through the factory connectors and harnesses, and adding some alternative and separately switched triggering schemes of my own (to parallel the batteries prior to start, to power the relay from a source other than either battery, etc).
I have pictures of that project also, but am not sure where those pictures are. It took me several years to get around to posting this one, so it will probably be a while before the follow up pictures appear.
One kind of crazy thing I did, that the picture above eludes a couple of times, is utilize BOTH the original LOWER brace as well as the replacement factory battery tray simultaneously, by modifying the lower brace to nest directly underneat the factory battery tray. It was my opinon that the lower brace's diagonal shape and metal thickness was sturdier than the OEM factory battery tray that was intended to substitute it. I also determined that having the combined strength of both took only a few slight modifications of the original lower brace. The picture show can't show how much stronger the result is, but it can show how visually seamless the result is.
As an aside, I also retained and reutilized the UPPER fender brace as well, which is shown more clearly now in a separate thread. http://chevyavalanchefanclub.com/cafcna/index.php/topic,136505.0.html
Hope this helps someone with a 2000-2006 GMT800 based GM truck who is considering replicating the factory wiring for 2nd battery installation in a gasoline engine application.