My 1970 Cougar XR7 Convertible

Hello everyone I am having trouble with my horn on my 1970 Cougar. Would someone tell me what the voltage VDC I should be measuring at the horn when it is pushed.
Here is my issue if I have just the passenger side horn connected And pull off the double wire on the drivers side horn the Passenger horn blows.
If I then connect the drivers side horn and the passenger side, they both do not blow? Connecting the drivers side only the horn does not blow also, But sometimes they do.It appears to be a ground issue?
I have replaced the drivers side horn and that does not work.
Does someone have an answer for what I should do?

Thank you
R70 Cougar