I had the same problem on my 1986, 560SL. I also traced it back to the horn slip rings. You can test the slip rings by removing the stearing wheel and exposing the brushes of the slip rings. The horn brushes are the two on the top not the double brushes they are used for the air bag. You can test with a jumper wire to the two brushes if your horn blows you have a short in the wheel slip rings. If you blow the fuse again it is shorted on the horn side.

I did not find a short and when I replaced the wheel every thing started working. I guess some debris had shorted the rings.
Hope this helps I can send you the diagram if you like.

Dave
86 560SL

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Dave