I'm not 100% sure, but I think the infra red remote was an option available only on 124 models delivered in Germany. Your post gives clues that you may be located in Germany. It would be helpful to know if this is indeed a car built for German delivery.

I'm not sure I can help you, but I will try...... If you lock/unlock the car by inserting the key into a lock (either front door or trunk, not using infra red remote) does the alarm arm and disarm?

J. M. van Swaay