After you have beaten the city champ (unlocked about half way throught the career) you will be asked by Karol to buy a 50% stake in his garages. To do this you must raise $1,000,000 dollars in total. After you have completed this mission the achievement will unlock. Now you can go and see Karol to beat the career. I would make this your priority over beating vehicle champs because once you've paid Karol all the cars and upgrades are free (expect unlock-only).
To get $1mil I started by doing some more of my missions, for example the tournaments and beating some of the kingpins (unlocked cars can be sold for quite a bit). I also sold all my vehicle except the one I use the most (the Ninja bike). From there I just did 'Welcome to Hollywood' in arcade mode. It is the shortest race, and with 7 opponents, traffic off and on hardest difficulty you can get about $5,400 every race.