All the Jeeps in Jurassic Park feature a CB antenna (but no CB radio has been seen) that is mounted to the rear of the Jeep with a 102″ antenna running up in a loop to the front windshield. The antenna mounts to the Jeep with a standard surface ball mount with a spring inbetween. The parts can still be purchased from CB retailers such as Radio Shack, although available online only.

Mobile CB Antenna Ball Mount Base
Model: 21-1115

Chrome-Plated Spring CB Antenna Mount
Model: 21-1118

102″ Stainless Steel Whip Element for CB Antenna
Model: 21-903
 

The antenna is mounted to the rear of the Jeep on the main body pan. The antenna is secured to the windshield frame to prevent flopping around.