Or, if you want to keep all the wiring inside the car to:
-run less wires under the hood
-have less wires to send through the firewall
-keep it from being as easy to cut the wires to the horn, which could in turn, prevent a theif from shorting out the siren at the same time.
Do a relay setup to trigger the horn when the alarm is activated. You should have an extra wire coming out of the alarm brain that sends a signal when the alarm is set off.
I have 3 possibilities of what color wire the horn wire in your Teg is:
-blue with a red stripe (least likely of the three)
-blue with a green stripe
-gray
You'll need a voltmeter to figure out which one is the right one in your car. The wire will show power at first, then go to ground when you honk the horn, wether the car is on or off.
All would be located in the steering column harness.
If you don't know how to install an alarm, or don't know how to make a relay work, this will probably be over your head. But it can be done without running more wires into the engine bay.