FYI, I and many others have this wired through the stock horn button. No issues unless you don't use a relay which I believe you have taken care of.

Technical specifications:
Sound Output: 139 dB
Operating Voltage: 12 Volts
Absorbed Current: less than 18A
Fundamental Frequency: 530 Hz & 680 Hz
Weight: 0.6 kg (1.3 lbs)
Dimensions: Approximately 4.8"x4.5"x3"
Reaction time: less than 90 milli-secon