Police say the body shop temporarily left the vehicle in front of the building on the 7300 block of Garden Grove Boulevard around 9:30 a.m.
"The business was preparing to deliver the vehicle back to its owner, so they moved it to the front of the building pending the owner's arrival," said Garden Grove police Lt. Ben Stauffer.
That's when an unknown suspect got inside and drove away in the car.
The shop called the owner, Nader Amirvand, to give him the bad news.
"I said, 'What do you mean, gone?' He said 'stolen.' I said 'are you kidding me!' My heart dropped, like you lost a baby," said Amirvand.
The owner is offering a $5,000 reward in hopes of getting his prized possession back.
The vehicle was described as a red 1963 Chevrolet Impala convertible with a white top and California license No. 7DG260.
Anyone with information regarding the investigation was urged to contact Garden Grove police at (714) 741-5899.
CNS contributed to this report.