Police in Shelby say the mechanic discovered the body Tuesday after noticing a strange odor.
Investigators say the mechanic realized the odor was coming from the trunk, opened it and found the box. When the mechanic went to put the box to the curb for disposal, he opened it and found the body.
Police say the car's driver is a 17-year-old girl. She is being interviewed.
Shelby Police Chief Charlie Roub tells the Mansfield News Journal it's not clear how long the body had been inside the car.