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2005 GMC Yukon XL 2500 8.1L W/4L85E.
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Finally found my issue and why my lights were doing what they were doing. I had installed a set of newer GMC style tow mirror's that have the LEDs on the front side and the glass side of the mirror for turn signals. That use your factory mirror plug for signals. A while back, probably 4 or 5 months ago. The previous owner of the mirror's had said he had custom wired in a resistor or diode or something to make the front LED strip on the mirror to both work as a signal and running light. What I found out was evidently one of the resistors or diode or whatever he had installed internally on the mirrors had went bad and was back feeding the turn signal power back down the running light power feed wire causing all the running lights to flash with the turn signals. I disconnected my running light power feed to the mirrors and problem fixed. Signals now work properly. I guess I'll have to order the Boost Auto Parts dual function tow mirror wiring harness so I can have my mirrors back to running lights and turn signals.
 

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Discussion Starter #43
I've always had the original unmodded OEM DL3 mirrors the truck came with installed. There is no running light feature in the factory mirrors. Mine are power fold, turn signal, heated, auto dim driver and memory. The trucks did not have the puddle light option from the factory.

Can you elaborate on this?
I disconnected my running light power feed to the mirrors and problem fixed.
 

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2005 GMC Yukon XL 2500 8.1L W/4L85E.
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I've always had the original unmodded OEM DL3 mirrors the truck came with installed. There is no running light feature in the factory mirrors. Mine are power fold, turn signal, heated, auto dim driver and memory. The trucks did not have the puddle light option from the factory.

Can you elaborate on this?
I installed a set of aftermarket new style tow mirrors. That come on all the new style gm trucks. Which have a signal light located on the glass of the mirror and on the front plastic face side. The previous owner of the mirrors had wired in a diode to be able to run the front face side led as a running light and still continue to use it as a turn signal. They worked for awhile like that after I bought them from the previous owner but evidently the diode failed and allowed to let the turn signal to back feed back down the wire used to hook up the parking light feature and was causing all the parking lights to flash with the turn signals. So in your case still running the factory mirrors this probably isn't your issue.
 

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One way or another there is voltage feed back like I stated earlier and the number of issues may very but I’m guessing everyone has done something aftermarket to the wire harness if not then who is having this issue without touching the wire harness so it will be easier to work this problem through
 

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I don't think it is in the rear taillight harness. The connections on the grey distribution block by the spare tire are:
blue=incoming power
grey=left taillight
black=right taillight
brown=license plate

With all four of these unplugged at the same time. It would make the front corner 194 led, on the opposite side of the selected turn signal flash at normal intensity instead of a faint flicker. Example, with the right turn signal activated the left front corner 194 led flashes at normal intensity.

Remember, with my issue it doesn't matter what turn signal I use. Either one will make the park lights and license plate lights flash. I think it has to be somewhere the left, right, and park lights share a connection or I have a pile of wires smashed. That is why I guessed at the flasher module.
You just diagnosed your own problem. The little gray box is the culprit. Replace it. They short out in older trucks. Another G.M. brainless part location. This is so common, I’m surprised at some of these answers.
at some of these clueless responses.
 

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Discussion Starter #48
You just diagnosed your own problem. The little gray box is the culprit. Replace it. They short out in older trucks. Another G.M. brainless part location. This is so common, I’m surprised at some of these answers.
at some of these clueless responses.
If the issue continues with everything unplugged from the grey box what would replacing the grey box solve? It would appear the issue is upstream from the grey box.
 

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Curious because I just ran into this same issue today, has anyone installed a LED flasher relay to replace the factory one. Or would the flasher relay not cause the feedback?
 
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