Philippe,
I am not sure what you mean by getting it to work without shorting out. The purpose of the prongs are to ground the horn circuit (in effect to short out the circuit) so that the horn blows. In this design, too long of prongs and the horn blows when you don't want it. Too short and the horn won't blow at all. The trick is to get the length just right so it blows with some pressure on the horn button, but not at other times.
Kent
