Why would the car roof light not work after you had an alarm installed?

Answer

Sure it could be, but not necessarily. Maybe the bulb burned out. Maybe the fuse blew? If your incapable of checking these out, take it back to the alarm installer and nicely explain what happened and they will probably take care of it even if they didn't cause it. Certainly don't accuse them since it may not be related.

Answer

WHENEVER INSTALLING ALARM SYSTEMS ONE OF THE FIRST STEPS THAT THE INSTALLER SHOULD TAKE IS TO DISABLE THE DOME LIGHT DURING THE INSTALLATION. IT CAN TAKE SEVERAL HOURS TO COMPLETE THE INSTALLATION AND YOU HAVE THE CAR DOOR OPENED TRIGGERING THE DOME LIGHT TO TURN ON. THE LIGHT CAN DRAIN YOUR BATTERY. THIS CAN EITHER BE DONE BY REMOVING THE FUSE FROM THE FUSE PANEL OR BY REMOVING THE BULB FROM THE DOME LIGHT SOCKET. THE ALARM IS USUALLY SPLICED INTO THE DOME LIGHT SINCE ANYTIME A DOOR IS OPENED THE DOME LIGHT IS TRIGGERED TO COME ON. THIS MAKES IT EASY TO ALERT THE SYSTEM THAT THE VEHICLE DOOR IS OPENED. DANO

When the alarm is installed, they splice into the dome light wiring as the last person said. This way when the door is opened and the dome light comes on the electric current acts as a trigger to set off the alarm. However when the doors are opened, they close a circuit on the negative side of the dome light. There is 12 volt power running to the light at all times. So the alarm has to be wired in on the negative side of the dome light as well. When they do this, only one wire running from the neg side is needed. It is possible that the installer cut the wire and wired the alarm in series and not parallel. What this means is that the alarm has been included in the electrical path for the dome light instead of just using the current.