I have a set, horns from a w126 ones lower pitched then the other and wiring/switch from a early w123, when wired the horns to the original horns and into the fuse box....people where complaining they were to pricey so I didn't relist them on ebay...
Mercedes Euro W123 W126 W115 W116 300D 300CD RARE Fanfare Alpine Horn Set | eBay