Marple Township police identified the woman as Nadia Malik of Broomall, who had been reported missing by her family on Feb. 10.
She was pronounced dead at 12:15 p.m. Police are working to determine how and when she died. An autopsy is scheduled for Friday morning.
Investigators were trying to determine whether her body was in the car the entire time since she was reported missing, including when it was towed, police said.
On Thursday afternoon, police cordoned off the area as passersby peered around caution tape at the Altima, still on 30th.
Police found no immediate signs of trauma, Lt. John Walker of Southwest Detectives said at the scene.
A black Nike duffel bag had been placed atop the woman's head, police said. The bag's contents - books and loose change - had been dumped on the backseat.
The passenger seat was "completely reclined" - "so as to minimize the ability to see inside," Marple Police Chief Tom Murray said in a statement.
Walker said a person on the outside "couldn't tell there was a body under the bag until you opened the car and grabbed the bag."
"It's a pretty disturbing case," he said.
This week, Marple police had asked residents to be on the lookout for Malik's Altima, last spotted at a traffic light on the 6400 block of Market Street in Upper Darby on Feb. 9.
On Thursday, a man called Marple police after reading an article in the Delaware County Daily Times on Malik's disappearance, police said. He said he "wasn't 100 percent certain" - but believed that a car he saw parked near 30th Street Station could be Malik's. It had been there for several days, he told police.
At the request of Marple's department, Philadelphia police checked on the car, then cordoned off the area and called Marple detectives.
Malik had been seen with Bhupinder Singh, 25, the father of her children, before she was reported missing, police said.
Singh was arrested Feb. 12 in Solon, Ohio, on a probation violation, police said.
Singh, who once lived in Upper Darby, has a criminal history in Delaware County that includes arrests for assault, police said.
The Nissan was purchased by Singh's father as a gift to Malik, police said. It was still registered to Singh's father, but was hers to drive, police said.
Singh has not been implicated in the death, but is awaiting extradition to Pennsylvania for questioning in the matter, according to Marple police.
Inquirer staff writers Mike Newall and Robert Moran contributed to this article.