I need some help, I hope.

I have a signal light issue that has me puzzled. I have the fuse that is labeled signal light fuse on the fuse cover, (not the same on a chilton guide) but anyway, I keep blowing the fuse during the operation of the vehicle. It can sit in the drive way with the signals on for an hour and the fuse stays in tact. The 4 way flashers work all the time, with or without the fuse. The odd part is the fuse blows with the signals NOT on and just driving down the road. I replaced the harness in the steering column, (had kind of suspected this being the problem as a possibility) and it has not changed the result. I have looked at the fuse panel and traced wires up to the harness and cannot see any obvious grounding out locations. Does anyone have any ideas before I dismantle the whole dash in the truck and start having to trace wires on this circuit?
Any good ideas would be appreciated
BTW this is on a 1991 F150 Lariat power everything single cab.

 

Look for a problem in the wiring going back to the lights. First place I would look is trailer wiring.

 

I have removed the trailer wiring harness and taken the electrical system there back to factory original (no plug in anymore)

 

also look at the bulb sockets in the lights themselfs.. could be corroded

 

Possibly your brake lights shorting do to only happening while driving , just a thought .

 

Thanks for the input all, after tracing circuits and wires from from to rear, I got down to peeling back the shielding from around the plug for my O2 sensor and about 3 inches back, the wires were frayed and grounding against each other. Guess I did something when I rebuilt my engine and damaged them��, but I can say "problem solved!" Thanks for all your thoughts!