The Oakland County sheriff's office says an employee of a property management company stumbled upon the body in the rear seat of the vehicle while doing a walk-through Wednesday at the house in Pontiac.
An autopsy was performed Thursday and found no trauma to the body. A cause of death has yet to be determined.
The body is believed to be that of a 49-year-old woman whose last known address was at that home.
Neighbors have told investigators that they haven't seen the woman who lived in the house and believed she moved away at least three years ago. They say the woman kept to herself.