I have a 1966 T5 conv with the dual horns. My brother has a 1967 coupe with dual horns. When we press the horn button with both horns connected they sound in a low volume. When 1 horn is disconnected the horn sounds louder. Both Mustangs do the same thing.Any ideas?  

If still factory horns, one should say low (66 right/67 left)and one should say high(66 left/67 Right)).  The diaphrams could be over adjusted causing more current flow into the tight horn thus reducing the overall sound. Do both horns sound individually?  

I had the same problem in my 67.  I had to end up replacing the horn assembly on the steering wheel.  They sound nice now.  My right was the low and my left was the high volume.  

Hi scruff6060 - jlg2002 and ak7mann have good questions and advise. Make sure your contacts at the steering wheel are good. U may have to replace these contacts if they area worn. Also, I read a MM article recently on horns that I believe said u can spray WD40 down the adjuster holes to keep the diaphragm lubricated so they work better.