if you claim that the guy installed a relay with the horns and pulled a positive wire direct from the battery then Im assuming hes switching the negative from the relay, The negative feed that is on the bridge of the relay is bad. Either supply it from the battery or use it from a dedicated ground point.
Another point to consider is that the factory alternator is about 35 amps or something, this is useless IMO as when the engine runs along with headlamps etc, it goes to full capacity, consider this, approx 10 amps is used by the fuel pump n EFI, headlamps are about 10 amps, 5 for the rest - you are already at max capacity at this.
The other point of failure usually seen in pk suzuki model wiring is the missing ground strap from the engine to the front of the car. Make sure its there.