The wikipedia article continues: "Lara Asaro, the girlfriend of Williams at the time of the crime, gave testimony that Williams had confessed to her... This is after she discovered evidence from the crime scene in Williams' car."
This is an important clarification: The police did not find the victim's property in Williams' car. Rather, Williams' ex-girlfriend, who was incentivized by reward money, claimed, more than 8 months after the crime, that she saw the victim's property in his car.
This is one of the the main reasons that the Innocence Project correctly argues that there is no reliable evidence linking Marcellus Williams to the murder.