What LED lights are you using? In normal use, if it's a combo tail/brake/turn lamp, are all LEDs used for tail then get brighter for turn/brake, or is there a separate bank of LEDs inside the lamp for tail vs. brake/turn?
Anyway, brake/tail on the GMT-900 platform is driven directly from the BCM. If not caused by a bad ground, I suspect you are seeing some kind of leakage current.
You may also be experiencing "hyperflash" when you replace tail lamps with LEDs. More here
https://www.gmupfitter.com/files/media/photo/537/UI%20Bulletin_81b.pdf
A 6 ohm 25W resistor placed in parallel (not series) with the LED brake/turn bulb will make the LED bulb appear like an incandescent to the BCM. This will cure the hyperflash without having to re program the BCM, but I'm not sure if it will cure the dim LED leakage issue. I've never liked using resistors because they get hot, and it defeats the purpose of having a low current LED bulb in the first place.
More info from from the GM Upfitters manual:
https://www.gmupfitter.com/files/media/photo/68/2008_LD_ElectricalPickupsChassisCabs.pdf
LED or Additional Turn Signals
Amber turn lamps on the NEW C/K Trucks are only used on the front of the vehicle. Rear lamps are combined stop/turn (red). Turn signals are controlled by the BCM. No flasher module is used. Adding additional bulbs will cause the BCM output to shut down resulting in no flashing. Replacing the front turn lamps with LED's will result in a rapid flash rate. This FMVSS required lamp outage detection is due to the change in lamp current through the BCM.
Two relays (left and right) need to be added for additional or LED turn signals. Power for the relays needs to go back to the UBEC. Turn signals are available at the forward lamp connectors. Information on these connectors is shown in Section C of the Utility Body Builders manual shown on the Upfitters website. The address of the manual is: http://www.gmupfitter.com/publicat/2007_BB/2007_LD_FS_CK_
Utility_Elec.pdf
