I have 1966 mustang and my left front blinker on my car doesn't work along with my horn. The rear blinker on the same side blinks and the parking light works, just no blinker.  Also, the horns don't work and it looks like the previous owner worked on this problem because of the tape on the horn connections.  It is possible that I have a bad harness?  

Check all your ground wires to insure you have a good ground at all locations.  Sand and/or brush off the connecting point on both the wire and the ground point.  

Get out a test light and check your horns for voltage when the wheel switch is pressed (the ford horns are really susceptable to failure or being overly tight).
The blinker signal comes from the turn signal switch and if the test light doesn't blink, that's your next step (do Soarings checks first though)