What car do you have? I don't remember if AU1 had speed alert, which may make them slightly different, but if it's AU2/3, here it is:
The wires from the buttons go to the instrument cluster, which intercepts the button presses and either performs speed alert functions, or "fowards" the appropriate signals to the head unit. There are two wires going into the head unit for this - both on the main connector plug. One is red with a black stripe, the other is orange with a black stripe.