To get bonus cars, get a medal in the corresponding event:
Ardent: Boxed In
Demon: Rod Rage
Kodiak: Monster Mash
Magnus: Heads Up
Opulent: Laps of Luxury
Outlaw: Toll 'N' Thunder
Phantom: Bayside Brawl
Roughneck: Trail Breaker
Streamline: Shore Leave
TEC: Heads Up
Vulcan: Catch My Drift
Warlord: Armageddon
Enforcer: Swift Justice
Guardian: Short Stretch
Hitman: Tommygunner
Honcho: King of Bling
Hookzilla: No Parking
Rampart: Raging Bull
Wraith: Killer Curves