Looking for circuit diagram for evaluation of a pulsed signal
I found a user manual at AMPIRE Car Alarm System for High-Speed CAN-Bus.


  N1/9 BROWN/WHITE – 200mA (-) Horn honk output: 
  
  This wire supplies a 200mA (-) output that can be 
  used to honk the vehicle’s horn. It provides a pulsed output when the security system is armed/disarmed 
  and in trigger sequence or in panic mode. In most vehicles with (-) horn circuits this wire can control the 
  vehicle’s horn without adding a relay. If the vehicle has a (+) horn circuit, an optional relay must be used 
  to interface with the vehicle’s horn circuit. 
  
  IMPORTANT!
   Never use this wire to drive anything but a relay or a low-current input! This transistorized output can only supply (-) 200mA and connecting directly to a solenoid, motor or other high current device will cause the module to fall


Judging by that this output will sink up to 200 mA. You could conceivably set an Arduino pin to INPUT_PULLUP and let this output drive it low to indicate it is active.

Personally I think I would use an optocoupler, in case there were unexpected voltages on that pin. You would need a series resistor to limit current through the LED in the optocoupler, say 1 k would be about right if you were connecting it to the 12V car system.

Vf = 1.2V (say)
Voltage over resistor = 12 - 1.2 = 10.8 volts
Current through LED = 10 mA
Limiting resistor = 10.8 / 0.01 = 1080 ohms


Now the output of the optocoupler can sink your Arduino pin, which you have configured as input pullup.

Write a test sketch to see what you are getting through it, but a multimeter should probably tell you, if it just flashes it slowly.



Suggested schematic: