Hi recently my alarm horn on my VW Golf stopped working cut along story short the fault lied with the alrm horn electronics which was replaced and now functions ok. I was currious so took the unit apart inside was a circuit board with a few capacitors etc a battery and speaker. The unit had three terminals which from a wiring diagram shows a 12 supply, earth and a data signal wire I know this is vage but what would the data wire signal be sending is this just a binary command possibly?

I can attach a picture tomorrow for you all to see.

 

My Sprinter has a similar setup.

You have a smart horn with a back up battery. It's self powered, and needs the data wire to tell it what to do.
If your car gets broken into and someone unplugs your alarm horn it stays sounding untill the internal battery dies. If you disarm your alarm it recognises the data and stops sounding.
That's a better setup then the conventional alarm siren that opperates off 12v.

I'm not sure if this is what you were asking but its my .02 cents worth.