Authorities said it appeared Sullivan and a 2-year-old boy were inside the car.
When the officer asked to look at a camera that was inside the car, he found a photo of a nude 12-year-old girl. The officer then found the girl under a tarp in the back of the car.
A search warrant said the girl told the sergeant that she went with Sullivan to the location all of the time because it was their "quiet place."
A local newspaper stated that Sullivan was the live-in boyfriend of the 12-year-old and 2-year-old's grandmother. It said court records showed the two kids also lived with their grandmother.
Sullivan has been charged with indecent liberties with a minor. He is currently in jail under $25,000 bond.