Brothers Michele and Carmine Comentale were driving on Monday through the mountainous Monti Lattari area, close to Naples, when the explosion happened, Il Sole 24 Ore reported.
They were left in a serious condition, while one of their brother-in-laws, Ciro Ruocco, was in the back seat at the time and suffered a head injury.
Lieutenant Colonel Nicola Conforti, from the local police force, said the explosion could not have been an accident.
Investigators are now looking into whether the bomb was placed by the roadside or in the car itself, Il Sole said.
The area is controlled by the Di Martino mafia clan, the newspaper said, which makes hundreds of thousands of euros each month by growing marijuana.