trying to add horn to factory alarm

Hello all! I have a 2002 Audi A4 with a factory alarm system. The siren is VERY quiet, so I was planning on splicing a relay into the siren power wire to power my air horns whenever the siren is sounding. This was supposed to be a simple project, find the switched 12v siren wire, T off of it with a relay, and run a line through that relay to my horns. Like most projects, it turned out to be more difficult than planned. The siren uses 3 wires. A Brown Ground, a Black with Yellow Stripe 5v, and an orange 12v. The problem is, all three wires are connected 100% of the time. I pulled the connector and plugged in my test meter, and all three wires are connected to their sources 100% of the time! With no changes in ANY of them between when the alarm is off, or when the alarm is on. Meaning, the ground is ALWAYS grounded. The 5v is ALWAYS 5v. And The 12v is ALWAYS 12v. My voltage meter shows NO changes ever! So the main question is what makes the alarm siren sound??? Could it be something with the amperage or a computer signal sent that a voltage gauge can't read? There are most definitely no other wires going into the siren. The siren makes two different sounds, a lower pitch chirp when you lock the doors, and a higher pitch screeching when the alarm is activated. This could be the reason behind the third wire. I want to tap in to the alarm screeching, NOT the chirp (meaning I don't want my air horns to sound when I simply lock the doors, only when the actual alarm is going off). The alarm and siren are working properly. Before I found out it was a three wire system, I thought it was simply a switched positive 12v and a ground. I was going wire in a 528T pulse timer, so the 1/2 second "door-lock chirp" wouldn't activate the horns, but the 2 second "ALARM!" screech would. Any thoughts about how this system works, or how I can T into the system? Thanks!

Can't be done, it's all data to the factory siren.

ARG! German car's are great about over-complicating everything. Well thanks for the info. Second plan could be moving the siren outside. And hide it up under the rear bumper. The biggest problem is the siren is buried in the trunk under all the trim and carpet. It was LOUD when I had the trunk apart and I was staring right at the thing, but put everything back together and close the trunk up, and you can't hardly hear it.