I have a 2009 F-150 and when I hook up the trailer, I have no left brake or turn signal. the regular lights work. checked relay and fuse and I still have the problem. any ideas?

 

Check the ground on the light. Make sure it is tight and clean

 

The left turn/brake light is one in the same. You could have a broken wire, a bad connection or even simply a bad light bulb. If you have marker/running light at that light fixture you can rule out the ground. Check the truck plug with a test light or meter to eliminate the truck. Also look at the truck plug and make sure the terminals aren't squished flat making poor connection.(7 pin connectors pins are v shaped) If that check out ok its got to be in the trailer.

Use a meter on the trailer plug and you can see if you have any connection on the left turn/brake light circuit. Is the trailer plug nice and clean brass? Using a meter will tell you if you have any connection.

Broken wire, bad connection(including ground) or bad light bulb if it's on the trailer side

 

Blown fuse. There is a fuse for each side.