'99F350sdf. My left brake/turn signal is not working. The #1 7.5a fuse(under hood) is good and there is power in/out of it. I do not have power to the connector that is under the truck bed by the spare tire. So I am thinking about running a new wire from front to back. So the question, Where does the wire go after it comes out of the fuse bounder hood). I can see four plug connectors under the fuse box, is it one of them?

 

I had to remove a connector in my trailer wiring because of a similar problem. I traced power to that connector but try as a might couldn't get it to pass through to the other side. Removed the connector and soldered the wires together - problem solved.

Just to the right of the fuse box there is a bundle of wires running up to the turn signal switch. The left turn signal lead should be light green with a white stripe assuming your 99 is the same as my 2000.

 

I lost my trailer brakes and chased it down to the pigtail: https://www.ford-trucks.com/forums/1...tle-piggy.html

 

if i remember right the 99's had 2 - 7.5A fuses for the trailer brake/turn signals. 1 was for the left bank the other is for the right. i dont remember which spot was which.. but i know one of them was in the #1 fuse location and the other was in the #3 location. Since you checked number one location try the other.

 

On my 2001, left is #1 and right is #3.