Brake light/ turn signal

START WITH THE BLUB.Turn and break are the same filament of an 1157 bulb. Unplug from TV and check TV plug. Disconnect trailer wiring from light. Plug in TV and check trailer wiring. Yellow and green are turn signal. Red is brake, white is ground.

What is the tow vehicle?
If it is the F150, check for blown fuse in the fuses under the hood.

Get a cheap 12 volt test light. Less than $10 and more forgiving when chasing tail light.

You need to make a good ground somewhere. Use a scrap piece of wire if necessary. Once you have the alligator clip connected to a good ground you can probe the socket first since you changed the bulb. You could easily have purchased the wrong bulb or not have it inserted properly. A simple test is to remove the same bulb from the left light and put it in the right side. Test your old bulb in the left side as well.

If you still have the issue then you should really start at the trailer plug side of the tow vehicle. Could be a blown fuse in the towing vehicle.

You need to be careful to not introduce a new problem while trying to diagnose this one. Take pictures as you go and print some out. Document what you did.

Start with a multi-meter or test light on the vehicle side of the 4 pin connector. If all is good there try cleaning the 4 pin connector. After that, a quick visual inspection of any wiring you can see for cuts or breaks. Then do as stated above, run new wires.

Broken wire somewhere. Easiest just to run a new wire. Butt connector (weatherproof) it at the light and at the 4 pin connector.

You can try to track down the break, but why bother. Running the new wire is easier.

Another possibility is the socket is bad, but I am betting on the wire.