cambia
displaywhennotempty="-Seleziona-"
in
displaywhennotempty=['-Seleziona-','-Seleziona cilindrata-','-Seleziona modello e anno-']

e
eval("son.options[0]=new Option(displaywhennotempty,valuewhennotempty)")
in
eval("son.options[0]=new Option(displaywhennotempty[following],valuewhennotempty)")

ciao