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Your description, I take it, is a problem with the wiring harness going from the driver's compartment to the rear lights. If that is the case, the normal color code for GM is brown for tail lights, dark green for the right stop & turn, yellow for the left stop & turn and light green for the backup lights. Any black wires should be grounded. The white wire in the trailer harness is for a ground when used on a trailer. It sounds like you have the green and yellow T&S wires shorted together somewhere! Some of the later vehicles use separate stop and turn bulbs... these would have a separate wire for the stop lights only, but I don't think your truck is new enough for that. Good luck!
Hook up the trailer to the vehicle. Under the rear bumper you will find a thick wire harness that runs from left to right or vica versa. Choose a location to strip open the harness to gain access to the different color wires. Open the trailer coupling. remove all wires and split them apart. Now start with say the right indicator light on the vehicle. Earth should be established via the grounding method of the trailer hook steel on steel of the vehicle. Connect any wire of the trailer to the now flashing right indicator light. Go through them all till the right indicator bulb flashes on the trailer. A good approach to determine which wire is active, is to make a test light from an old bulb connector some have already two wires connected ie; pos neg. neg the tester onto any part of the vehicle chassis and the pos to the now live wire. Go through them all till all wires meet up. Now go on and extend the harness by means of soldering properly and insulating that wire from the main harness to the female fitting of the trailer connector. At each connection on the fitting fit on properly the different wires and do the same to the male connector from the trailer so that all lines up inside the two connectors. Good luck
You need a "test light". Then you turn on the circuit you want to wire in, tail lights, brake lights, blinkers and poke around the back of your light sockets until the test light comes on. Then you splicew into that wire and run it to your plug in. If you don't know which wire is what on the trailer, hook it up and with the wire fastened into the truck harness, try it on the trailer harness wires until you get the proper lights working. Process of elimination. Hope this helps.
The brake lights stay on even when the vehicle has been restored to the
"before" condition?!? Now that's strange. I'm assuming the brake lights
worked properly before you did anything with the trailer hookup. Is that
correct? Do the brake lights work OK when a trailer is hooked up to the
trailer lighting harness? Regardless, if the brake lights stay on all
the time, there is a continuous 12V to the bulbs to ground. You have a
wiring short somewhere in the vehicle. Have you checked the wiring for
bare spots (caused by rubbing) or cuts in the wires.
Since the brake lights won't turn off, you have created a continuous
path of 12V through the bulbs to ground, regardless of the brake switch
operation. If you did not use the plug in type of wiring harness to
attach the "trailer plug", then I would systematically undo each wire
that you attached to and see what fixes the problem. I assume you know
that the Blazer has a trailer connection plug-in location tucked up
under the rear bumper. You simply unplug the mating harnesses, plug in
the trailer wiring connector, plug in the disconnected harnesses and
wha-la you should be good to go.
Under the frame, your Sierra has a trailer harness. Locate it and hook up your trailer harness to it, every Sierra has a factory harness hook up in it, here is a link of your wire harness should be located under your Sierra.
Thank you for using fixya and good luck.