The baby, whose umbilical cord was still attached, survived with minor injuries and was hospitalized in serious condition Friday, reports Joan Murray of CBS station WFOR-TV.
"He weighs 8-pounds, 2-ounces, and I have to tell you, I saw him, he looks beautiful, he's a little fighter, full of life," Veda Coleman-Wright, of the Broward County sheriff's office, told WFOR-TV.
Broward County's sheriff said Friday that he hopes that the people who threw the boy out of the car are feeling remorse and will come forward.
Investigators are trying to identify the couple that threw the boy alongside a busy street in North Lauderdale, 13 miles northwest of Fort Lauderdale, on Thursday afternoon, Sheriff Ken Jenne said. The car then sped away.
"The first thing we want them to do is to surrender themselves and come in," Jenne said on CBS News' The Early Show. "We really need to talk to the people who abandoned this child in this fashion."
A woman stopped and found the baby, believed to be less than an hour old, inside a small plastic bag. She scooped him up and took him to a nearby sheriff's office, and he was transferred to Broward General Medical Center.
"She stopped her vehicle to rescue what she thought would be a cat or a kitten," Jenne said. "She got out of the car opened it up took a plastic bag off the head of the individual and discovered the newborn."
The good Samaritan was "very distraught, very upset," Jenne said. The woman's name was not released.
Investigators are searching for a white, older-model large sedan that witnesses observed leaving the scene. The baby's rescuer told investigators she observed a man and woman arguing inside the vehicle. But she saw only the back of their heads, Jenne said, and could not provide a good description.
Police say the couple didn't know, or ignored, Florida's safe haven law, which allows mothers to give up their babies at a safe location.
"We need to use this incident to remind people that you can always take the child to a hospital no questions asked if you do it within that three day period," Coleman-Wright said. "There are safe house, there are options out there for you."