Not sure if this applies, but do you have a hi/lo or city/country horn switch? The 560 does, looks like a sideways exclamation mark, using the switch you can get a tinny sounding little horn or a full blast loud horn.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.