diode isolation and cavalier door trigger

I've got a 4dr 2001 Cavalier I'm putting an alarm in.  For the door pins, I see they are separate.  GREEN/ black wire for driver's door only, Lt. Blue for the other 3 doors.  The install guide says to tap both, diode isolate each and take to -ve door trigger input on alarm.  The door pins are 12v when closed and 0v when open.  If I put a diode on each of the two door pin wires, with the annode facing the door pin wire and the cathode facing the alarm input wire, the only way I will ever have 0V on the cathode side is if both inputs are at 0V.  This only happens when BOTH doors are open.  If only one door is open, the alarm input stays at 12v, as the wire from the door that is closed is at 12v and continues to pull up the alarm input to 12v.  How do you keep the two doop in switch wires isolated, yet allow either one to pull the alarm input down to 0V?