'01 Ford Ranger door triggers

 Hello,

I am installing an after-market alarm in my Ford Ranger. This alarm has  positive and a negative door trigger wires. I need to know if I will be using the + or - trigger wire on the alarm? Also, what color wire (and where is it located?) should I be using on the Ranger to tie into that will trigger my alarm when the dome light goes on when the doors open? (So that when my door opens the dome light will go on  it will send a trigger to my alarm)  Does the alarm wire need a diode after tying into the dome light?

Thanks again if anyone can help,

Larry