Astra door trigger wires?

I have a 98 jeep cherokee and I got an astra 777 alarm and the wires that ask you to hook them up as the door trigger...well i figured i could just attatch it to the dome lights since they come on all the time but my alarm disregards the doors because it says they are open...when i checked the voltage on the dome light wire it says its got 12 volts even if the door isnt open which would make the alarm think that the door is open

so what should I use as the door trigger? or should I redo something?