La Joya police spokesman Joe Cantu says the car door bounced against a tree and slammed shut on the man's head, cleanly severing his ear.
Police and Border Patrol agents found four of the six passengers who fled the car after it came to a stop.
Authorities also discovered four people in the car's trunk who were thought to be illegal immigrants from El Salvador and Honduras.
The driver was taken to a McAllen hospital for treatment.
