I agree with ard. The steering wheel contact is not why the fuse is opening at this point. Either:

(1) The contact in the steering wheel closed for some reason which picked up the horn relay and energized the horn. Then, after the horn sounded for an extended period of time, the horn failed internally, shorting the circuit and causing the fuse to open, or

(2) A contact in the horn relay was closed for some reason which energized the horn. Then, after the horn sounded for an extended period of time, the horn failed internally, shorting the circuit and causing the fuse to open, or

(3) A fault in the horn power supply wiring initially caused the horn to sound for an extended period of time which caused it to fail internally and cause the fuse to open. The damage to the horn and/or the power supply wiring fault continues to cause the horn malfunction and opening of the fuse, or

(4) A fault in the horn control wiring could be be causing or contributing to this mal-operation.

As you are experiencing poor function of the horn now, there is little doubt in my mind that it is damaged and is causing the fuse to open. That is only half of your problem, though.

The fuse is supposed to protect the load; in this case, the horn, and will only open if the load is drawing too much current. The horn is an intermittent type load and itself not designed for extended operation.

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8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim