87 SDL Factory Alarm

[the_good_fellow]

I was at the golf driving range and left my car unlocked for about 2 hours. After that I was on my way to a golf course, ¼ miles down the road at the light a guy in a 4-runner roll down the window and asked me if I stole it. I said "NO" and he told me that my headlight was flashing. he said it looked like the alarm was activated. so I pulled over and reset it with the key in the driver door. I tested the alarm with window rolled down and reached in to open the door. It works fine but the horn doesn't. I'm wondering why the horns never go off and how it is onnected??? It has a aftermarket stereo installed by PO. Is there a relay somewhere?

thanks!

BJ