Making lights Dim with the courtesy lights

My cousin has a 2000 Firebird and we hooked up red mini neons under the dash and he wants it to go on when he opens the door and have the same functions of the courtesy lights. I searched all the wires and found that a white wire has power when the lights are off but not on and a blue wire with a white line has power when the lights are on but it says on when the car is on. When one wire has power the other doesn't and I know that these wires along with a black wire are used for the factory courtesy lights. Does anyone know of a relay or something to make this work?