When the officers opened the trunk, they found the body of a woman who may be 20-year-old Lisa Marie Bromiley from Lancaster.
Relatives and friends had posted fliers in the area looking for information on her whereabouts.
Bromiley was last seen leaving a friend's house in Palmdale on Saturday.
Police have not identified the woman or said who the car belongs to, but a family member said the car belongs to Bromiley.
Police have not said how the woman in the trunk died, but said the body appeared to have been in there for several days.
Anyone with information is urged to call the Los Angeles County Sheriff's Department at (323) 890-5500.