Alarm help please

[Eiknujrac]

Ok so this is the deal:

I own and drive a 1991 300e, and have for the past couple of months been trying to diagnose why the alarm in my car is not working at all. I have confirmed the presence of an alarm, from the white alarm horn on the drivers side behind the headlight, the hood switch, and the module under the passenger side footwell. But for the life of me I can't figure out why It won't work. Would the installation of an aftermarket head unit have anything to do with the non-working state of my alarm? From what I read the stock alarms were fairly well tied in with the stock radios.

Any ideas would be appreciated, i've checked most fuses and connections, but anything helps.

Thanks

-E