2004 Yukon 5.3 keeps running when key is out... SOLVED!

I really could use some help with this one. The battery drained overnight on 2 random occasions, which led me to the NAV unit I installed (the colors for the switched + and the constant + are reversed on the newer stuff). I have quick taps installed in the ignition harness for a remote start I'm ordering in the future, so I used the harness. As I was testing the unit (truck running), the wire for ignition 1 (pink) arced out on bare metal. Sparks flew and the truck shut off.

When I turned the ignition back on, the only light that came on was the battery light, so I ordered a new ignition switch from the parts store. Then using a jumper wire and the quick taps, I turned the ignition on and started the truck. Ran perfect, until I got to the store and turned the key off and the truck still ran. So I pulled the fuel pump relay and ran it out of gas. There was no change with the new ignition switch.

I ended up replacing 3 maxifuses in the fuse box under the hood. Then everything worked normal and I was relieved, until I turned the key off and the engine refused to shut off.

I pulled the fuel pump relay, and let it shut off. I did this with the key still "on", and when I turned the key to the off position, the radio and dash shut off like it's supposed to. I also tested the "retained power" circuit by leaving the door closed and reaching through the window to pull the key out, then opening the door. Works perfect. I don't know what to do or where to start looking. I've attached a diagram to the fuse box under the hood. If only I knew what all those abbreviations stood for.

I think you need a service manual or an alldata DIY subscription.

Do you have your owners manual? There’s a description of what all the abbreviations for the under hood fuse panel and interior fuse panel fuses and relays are for.

No I do not. Even if I did, I'm not certain the manual would indicate which one actually kills the engine. Or if it's a combination of fuses/relays.

Seems like if you pulled one of the PCM fuses that would kill it, but I don't know for sure. You have bigger issues than just getting it to temporarily turn off. Might be worth the AlldataDIY subscription as @exp500 mentioned, or an appointment with a repair shop that is good with electrical issues.

You may want to check around and under the underhood fusebox and the engine compartment for rodent damage, just a suggestion.

Sounds like a stuck/welded relay. But many other possibilities too. This will show the circuit/ module data, and in conjuntion with service manual, almost makes it easy, at least logical.
Vxdiag VCX Nano for GM/Opel Diagnostic/ECU Programming Tool

I just went out and bought new fuses for the under hood fuse box. While I was at it, I ran the truck, then pulled the Ignition 1 relay. It died immediately, like it's supposed to. Reinserted the relay with ignition still on, had a few tiny sparks when I plugged the relay in. Did it again with the ignition off, no sparks, which tells me there is no power going to that relay when the key is in the off position. I replaced the relay with a new one, but it didn't work. At least I'm getting somewhere.

You really need to improve your tactics. Sparking things around computer citcuits is a very dangerous practice. Usually quite expensive. Might want to read up about that sometime.
There are many service manual.pdf available online free if you want to take the time to sort thru dead links.Ebay usually only $30 or so.

I've already got an alldatadiy subscription. What I was doing using a jumper wire to "hotwire" the truck to run was probably more dangerous than putting a relay back where it belongs with the ignition on, but I'm replacing all fuses today so, we'll see if it makes any difference.

Glad to try to help ya, but not sure exactly where you are in system and what you have done.
Since you have the manual, post the pics of it and note where your taps are. Then we are all looking at the same data.
There are some really good sparkchasers on this forum. I'm sure you will get an answer to your problem(s).