7 PIN WIRING

Yellow should be left turn indicator.....check that.
Stick a multi-meter on the yellow coming out of the truck end.

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Are you getting output from TV?
One of the reds could be for brakes.
Yellow should be left turn.
wiring diagram

check the ground on the left light could be loose or corroded. Also check any splices in that line for corrosion. Fixed a guys lights when they would not work, almost all splices were corroded as was the ground.

HBJBIRD wrote:
Replacing my 7 pin plug on my enclosed trailer. Wires from the trailer are: 2 red, 1 brown, 1 yellow, 1 black, 1 green. The 7 pin extension has: 1 red, 1 brown, 1 green, 1 yellow, 1 black, 1 blue, 1 white. Hooked up per instructions. Everything works except left blinker ? Any help is much appreciated. Thanks.

Per what instructions?

Since the brake light and turn signal are on the same wire, if the left brake light works but not the left turn signal, then your issue is likely with the tow vehicle, not the trailer or the plug.

The link posted above by enblethen is a great one; scroll down to the 7-pin plugs to see what should be going on and get out your multimeter.

7-Way Trailer & RV Plug Diagram
https://www.ajtnt.com/7-way-rv-plug-diagram/