2017 Tahoe SSV rear flasher issue

[AlphaZulu]

I have an ex-NPS Tahoe SSV that was stripped prior to sale. I found the wig-wag wire between the front seats and got the front wig-wags working, but nothing happened to the rear lights. After about a year of regular use the rear lights began intermittently flashing alternating brake lights and reverse lights without any input from me. I can't seem to control the flashing but after digging into the passenger side interior rear paneling I found a blue wire that when +12v is applied to it there is a noticeable relay click and both sides stop flashing, but I also lose turn indicators and brake lights.

Does anyone have an idea where I can start digging in to either disable, or preferably control the rear flasher function? I need my indicators and brake lights back obviously.

 

[NPS Ranger]

I'm not an upfitter but since you bought an old NPS bomb I thought I'd dive in. Have you looked at the GM upfitter manuals available on their website? For your model it says:

Option 6J7 provides a headlamps high beam flashing module, rear lamps flashing via the Body Control Module (BCM) and a control wire for customer-furnished switching to turn the module on and off. The flasher control wire is part of the blunt-cut upfitter harness coiled under the instrument panel in the front passenger side foot well. The flashing module is located on the rear edge of the right hand upper radiator support.
The headlamp flashing module is activated by the application of 12 volts to a dark green wire in the upfitter harness connector labeled 6J3. When the flashing module is turned on, the module sends a signal to the BCM which alternately flashes the stop lamps and backup lamps at the same flash rate as the headlamps.
Activation of the headlamps flashing and rear lamps flashing can be separated by opening the blue/green BCM circuit at the flasher module connector, C196-F, and applying a customer-switched ground to the blue/green wire in the upfitter harness which is located under the center instrument panel. Power to the green flasher module activation wire must be off for the rear lamps flashing only.
Warning: BCM will be damaged if 12V power is connected to the dark-blue/yellow wire.

On this model the flasher module controls the headlights flashing, and interfaces with the BCM to trigger the rear lights flashing.. Sounds like someone separated the functions at that flasher module connector, I'd look into re-connecting it to the BCM. Also, the fact that you can get rear flashing only by grounding a wire in the upfitter harness suggests you might have an intermittent short to ground on that wire.
The blue wire that kills your brake lights and turn signals sounds to me like a "rear light cutoff" relay activation.

 

[AlphaZulu]

I'm not an upfitter but since you bought an old NPS bomb I thought I'd dive in. Have you looked at the GM upfitter manuals available on their website? For your model it says:

Option 6J7 provides a headlamps high beam flashing module, rear lamps flashing via the Body Control Module (BCM) and a control wire for customer-furnished switching to turn the module on and off. The flasher control wire is part of the blunt-cut upfitter harness coiled under the instrument panel in the front passenger side foot well. The flashing module is located on the rear edge of the right hand upper radiator support.
The headlamp flashing module is activated by the application of 12 volts to a dark green wire in the upfitter harness connector labeled 6J3. When the flashing module is turned on, the module sends a signal to the BCM which alternately flashes the stop lamps and backup lamps at the same flash rate as the headlamps.
Activation of the headlamps flashing and rear lamps flashing can be separated by opening the blue/green BCM circuit at the flasher module connector, C196-F, and applying a customer-switched ground to the blue/green wire in the upfitter harness which is located under the center instrument panel. Power to the green flasher module activation wire must be off for the rear lamps flashing only.
Warning: BCM will be damaged if 12V power is connected to the dark-blue/yellow wire.

On this model the flasher module controls the headlights flashing, and interfaces with the BCM to trigger the rear lights flashing.. Sounds like someone separated the functions at that flasher module connector, I'd look into re-connecting it to the BCM. Also, the fact that you can get rear flashing only by grounding a wire in the upfitter harness suggests you might have an intermittent short to ground on that wire.
The blue wire that kills your brake lights and turn signals sounds to me like a "rear light cutoff" relay activation.

That makes a lot of sense. I read through that section a bunch of times while doing my install/setup, but I think I finally get what I'm looking for.

I didn't think about the blue wire in the back being a stealth mode kill switch, but that makes sense too. I may have to try to utilize that.

Thank you for you input and suggestions. Hopefully I can get this intermittent issue resolved once and for all.

 

[NoLimitSquads]

Does your Tahoe have an auxiliary battery? I have seen issues with the rear tail light flash coming on intermittently due to a failing Auxiliary battery. Otherwise, look for the blue/green wire located under the center instrument panel, and make sure its not shorting anywhere. And check the wire where they cut it at the front flasher module, and make sure that it isn’t shorted there either.

 

[AlphaZulu]

Does your Tahoe have an auxiliary battery? I have seen issues with the rear tail light flash coming on intermittently due to a failing Auxiliary battery. Otherwise, look for the blue/green wire located under the center instrument panel, and make sure its not shorting anywhere. And check the wire where they cut it at the front flasher module, and make sure that it isn’t shorted there either.

It does have an auxiliary battery which I run all of my equipment off of. It seems pretty healthy to me (doesn't drop below 12.3v even after I leave equipment on for an hour or so without the engine running), but I'll look into that.

After NPS Ranger's comment I finally found the flasher unit. It had broken off the rivets at the upper radiator mount, and someone had zip-tied it to an A/C hose under the airbox. I reattached the wire loop at the flasher and can now command the rear flashers on with the front. Does the blue/green wire still turn the rear flashers on independently from the fronts when this wire is not broken? I'd prefer not to have to dig into the center console to find that wire if I don't have to. There is a lot going on in there.

Thanks for giving me another avenue to look in to. I appreciate any and all help with this machine.

 

[NoLimitSquads]

I couldn’t answer the part about if that wire still activates the rear when they aren’t separated. I have always separated them for independent control. But I think that the front flasher module just outputs a ground signal to the bcm to activate the rear. So it would make sense that the bcm input goes both to the front flasher module, and to the console. So if the problem persists, it could still be that wire in the console( or anywhere else for that matter) shorting. I would just kind of scan the vehicle all over for wire harnesses that might be pinched or rubbing on sharp edges.