citydriver

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Check for the wiIring harness that runs over the transmission bell housing to the back of the right (passenger) cylinder head. Make sure it didn't get pinched between the trans and the engine block. Also make sure the ground on the back of the head is secure/tight.

Tail gate light bars sometimes mess with the lighting system also, if not installed correctly, or even the control board for the light bar goes bad without you realizing it. I would start by returning the stock bulbs into the tail lights and recheck. While you are in there take a good look at the lighting harness and check for damage. I don't mean anything by using the word "cheap" it's just that sometimes when you put LED bulbs in place of standard bulbs, the resistance values of the circuit changes and causes the BCM to miss read and causes issues with it's intended operation. It usually takes more research and parts to correct it. The electrical systems on newer vehicles can be quite the pain when doing even simple mods. Good luck. I need to go into work now.

Sometimes cheap LED bulbs or tail lights will cause feed back issues with the BCM. Have you done any modifications that may have caused un intended circumstances? Just throwing it out there. I work really weird hours, and sometimes cant reply for awhile, but some on the forums may be able to help with more information. This link may help you better understand how the electrical and lighting system works. Go to electrical and look for description and operation https://www.gmupfitter.com/pages/body-builder-manuals.

Depending on the Model and year of the truck , there may not be power to that wire unless the ignition switch is in the "on" position. Almost ALL lighting functions are controlled by the BCM (Body Control Module). You can get an accurate description of operation and wiring diagrams on the link https://www.gmupfitter.com/pages/body-builder-manuals.

It's been awhile, but if I remember correctly the blue and red wires are taped to the trailer wiring harness near the under hood fuse block. The red wire is for the battery +12v terminal on the 7 way trailer connector. the end near the fuse block should be attached to one of the studs on the fuse block and you may need to put a fuse in the block. The decal under the fuse block cover should tell you which slot to use. The blue wire is the electric brake wire that goes to the 7 way trailer connector, this is where your trailer brake controller wire (usually blue) gets connected to. Check this link https://www.etrailer.com/faq-cbc.aspx

If there was a trouble code set, and the Check Engine Light was on, the factory remote start will not even attempt to crank the engine. The lights may flash, but as soon as it "reads" the code, the computer will shut down the process. If it will not start with the key, but still cranks, there is another issue that will have to be diagnosed. Check all of the fuses. Check the battery connections. You should be able to hear the fuel pump turn on and run for a second or two when the key is turned to the "run" position before you try to start the engine. These are just the first couple of things I would check.

Just something you guys can check that I've run into a couple of times. If the fuse is good and the contacts in the fuse panel/block are clean (corrosion is an issue at times in the under hood fuse block, especially in areas where road salt is used in the winter). Check the main wire harness connector under the seat. Check for power AND ground connections on both sides of the connector (sorry, I don't have pictures). I have found problems with the main ground losing connection due to corrosion. If it's corroded, that damn connector is a royal pain to get apart. If it was loose and caused arcing, the connector body may also be melted. But it's usually that main power connecter under the seat that causes the issues if only one seat is not functioning.

There is a relay, but it's not serviceable, it's a PCB relay mounted to the board inside the under hood fuse block that is triggered by the BCM after it receives a signal from the transmission control module. But I would check the fuse and the fuse socket / connections. I have been noticing some corrosion on my under hood fuse block connections lately. On both sides, the top you can see and the underside that you can't see. I basically removed the under hood fuse block to clean and inspect ALL of the connections for the fuses, relays and harness connectors under the block, then used a plastic bristled brush and applied a thin coat of di electric grease to all of the contacts. I did this as a preventative measure to help keep the corrosion from getting out of control. Hopefully that's all you will find is a dirty connection. Otherwise you will need a Tech2 or similar to check the signals from the modules.