if the switch looks like a sideways exclaimation point, it switches the horn from "city" to "country" mode. Flip it, press the horn and see. As far as the owners manual goes...I have a 1987 560SEL which I am sure is 90% the same as a 1990. If you can't find a manual and you just want to know what's what, let me know and I'll photocopy mine and send it to you.
Justin