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Dead short in the horn, or, short to ground in horn feed wire.......

As stated above, the car has a dead short in the horn or its wiring.

Before you go looking for the short, study in depth the horn portion of your car's wiring diagram. Unlike other circuits in the car the horn circuit is "hot" all the time with the horn push button completing the earth/ground connection. This means thinking a little differently during troubleshooting.

Recently I had the same issue. Firstly, measure the connectivity (resistance) of both feed terminals on the horn against the mounting point/stud. A twin feed horn (as described above) does not earth through the mounting (bracket).
My horn had a short circuit, so I ordered a new one (http://www.minispares.com/product/Class ... .aspx|Back to search) which turned out to be a nice neat black Lucas horn.

That makes sense as on the original horn it had two feeds the one I got from ebay only had one maybe that's the problem like the posts say the wiring is terrible it all needs new connections and cleaning up may have to get a twin feed my mistake I assumed all horns were the same I do remember earthing one of my minis a mk3 on the bracket il order a twin feed horn and put new connectors on good news today bled the clutch and she moves so giong bleed the brakes tommorow waiting for an exhaust clamp to come thro and new ht leads thanks for the info everyone great help