Horn Issue - no beep

[TS69GT500]

69 GT500,  All stock, wiring etc.
Problem with the horns.
If both are plugged in they produce no sound.
Have to unplug the hi-pitch horn to get the low-pitch to work.
If the low pitch is unplugged the high pitch does not work.
Both horns were tested independently off the car and operate properly. 
Correct me if I am wrong but the Mustang does not use a horn relay.
I'm not very strong on electricals.
Any suggestions?
Thanks
TonyS

[Bob Gaines]

I would first check all of your connections (base of column headlight harness etc.} including the horn contacts on the bottom side of steering wheel and of course the way the horns are grounded on the radiator support for solid contact. It sounds like you are not getting enough current and when both are plugged in the circuit can't draw enough to make both work.

[Bob Gaines]

I should remind that the horn bracket has to make a ground connection to bare metal of the radiator support. Paint is a insulator. The serrated edges of the horn bolt or serrated flange nut sometimes does not dig through the paint enough to make a solid contact. 

[TS69GT500]

Solid info to start on.

Thanks Bob,
Much appreciated.

TonyS