To unlock this achievement you will need to deliver 30 vehicles to Stevie's garage. About three quarters through the main story line you will start to get text messages from Brucie's friend Stevie. He will ask you to deliver these vehicles giving you an image and a description/location of where the car is located. Here is the complete list of vehicles:
Banshee
Bobcat
Buccaneer
Cavalcade
Cognoscenti
Comet
Contender
Coquette
DF8-90
Dilettante
Dukes
Faggio
Freeway
Huntley Sport
Infernus
Intruder
Manana
Moonbeam
NRG 900
PMP 600
Patriot
Rancher
Rebla
Sabre GT
Sentinel
Sultan
Super GT
Turismo
Voodoo
Washington