84MB500SEL
I would be happy to help you if I knew ANYTHING about Euro cars. I do know that I once worked on an 84 280SL and the A/C looked like yours and it was an aftermarket add-on. I have looked at the wiring for a car like yours except a US version and the horn and A/C have nothing to do with each other that I could find. The way to fix this type of problem is to take one of the problems (I suggest you start with the horn) and ignore the other. In other words, one thing at the time. When you fix the horn, if the blower still doesn't work, then work on that.
Maybe some of our EURO friends can help if you ask specific questions. For instance, ask "what else is powered by the horn fuse". If you get an answer, then you can check those things to see if they work or not. Let us know the answer and we can give some guidance with those facts.
I just looked and on a US 500SEL Fuse 14 feeds power to the horn switch in the steering wheel. The other side of the horn switch goes to the horns. The other things on Fuse 14 are the cigar lighters, rear window defroster, rear seat heaters and the trunk light. I have no Idea if your car is the same but you can check and see if this information applies to the Euro car.
Sorry I can't be more help.
Paul

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84 500 SEL (307,xxx miles)