Mr. Katz was matched with his advertisers by FreeCar Media, an advertising agency in Los Angeles that claims to have a database of more than a million car owners who say they are open to wrapping their cars in ads for a fee, said Drew Livingston, president of the company.

The sponsor also pays as much as $5,000 a car for the wrap job. Generally, a car can qualify if it has enough surface area for a sizable ad and is no more than five years old.

“A company like Procter & Gamble will come to us and say, ‘We have a new and improved Tide, and our target is stay-at-home moms with two-plus children who live in these 20 markets,’ ” Mr. Livingston said. His company then finds drivers in that demographic. “We feel that when you can wrap a mom’s car and get it to her P.T.A. meeting or Curves gym, you’re getting the acceptance from her social circle.”

The company either gives its brand ambassadors free cars or, more often, pays them as much as $800 a month. In the last seven years, FreeCar Media has hired about 7,000 motorists, who are instructed to park outside whenever possible, refrain from smoking, littering or swearing in their vehicle, and to attend a monthly influencer event where they hand out samples or coupons. They also have to send reports frequently with photographs to show where their cars have been.

People whose cars were wrapped with ads for two Coca-Cola products — Planet Java, a bottled coffee, and Vault, an energy drink — were cautioned against sipping Pepsi products behind the wheel. Nor could they park at restaurant chains like KFC or Pizza Hut that serve Pepsi exclusively, Mr. Livingston said.

Another FreeCar participant, Jerome Harris, 22, was in his junior year at Temple University last year when he had his Nissan Altima wrapped for a promotion for Trolls, the endomorphic dolls with Don King hairdos. He earned $500 a month while his car was wrapped; in addition to driving around while on his best behavior, he was required to hand out Trolls pens to fellow students during finals week.

“We weren’t allowed to have alcohol in or around the car, or use profanity,” said Mr. Harris, who now lives in Brooklyn and is a few credits shy of a degree in advertising. “When you’re out, you’re supposed to be representing the brand.”