2006 Ford F150 left turn signal not working, 3rd brake light stays on?

I have a 2006 F150 King Ranch.

All of a sudden my left turn signal stopped working and stays solid. The right signal works fine. The 3rd brake light remains on as well.

When the headlights are turned off, everything functions as normal. When I turn on the headlights is when I have the problems. The signals both work with lights off.

All lights are working.

Could it be the MFS? Short? Fuse?

My explore had a similar problem. It might be the election wires or fuses, my truck had a wires rigged and they came lose so all my lights started acting up like your truck. I touched the wrong wire and fried the electronic system and blew a fuse and took two days and 5 dollars and fixed it. So look at your fuse box and see if you blew a fuse if that is not it go get your wires checked to be sure

Sounds like a wiring circuit is backfeeding power to the left turn and third brake light when headlights/taillights are on. Could be a headlight/turn signal issue or frayed/corroded connections at wiring connectors in the circuit. If there's corrosion in a wiring junction plug connection, the current may jump from one terminal to another for the left turn/third brake light from power for the taillights… Or melted /frayed wiring at connectors, etc… Or someone hooked up a trailer harness improperly and is jumping power to turn signal/third brake light when headlight switch is on. I'd check all the harness wire connections first… Check all plug connectors in the taillight harness for corrosion or frayed wires touching each other and clean/repair as needed if so… And check for issues with trailer harness if so equipped. Use a test light to track if power is improperly getting to left turn and third brake circuits at harness connections… Back track to headlight or turn signal switch circuits if needed.

You may need a wiring diagram to trace wiring circuits, locate connection plugs and wire colors… Part stores sell Haynes or Chilton repair manuals that have wiring diagrams or use online sites like Eautorepair.net or Alldata.com which have diagrams and factory repair info… Small fee.