The car horn is just a button on the steering wheel, one end of the +12V power supply (through the horn special insurance over), the other end is a line of horn, horn is grounded at the other end of this, press the switch, switch to the 12V through the button on the horn again, forming pathway, ring the.
If it is to other equipment installed on the steering wheel, to take power from here, the +12V line can be positive, if you do not know what the root is, can use the multimeter to check on the ground, there is a 12V voltage (note the battery voltage is not actual 12V whole car out, generally will be about 13V, a car is more than 14V).
The negative well in the car, the whole car ground is, as long as can find the nearest ground position draw a line, can be used as anode.
Also note that if the device power consumption is large, the current margin must take into account the original car insurance insert is enough, or horn and additional units exceed the current limit, may burn insurance, finally the device does not work, the speaker does not ring. If the current margin is not enough, it is not recommended to replace the larger insurance directly, because the original wiring harness design is designed according to the size of the insured insert. To put it simply, electricity from this position is not recommended for loading too much power.