Hold LB and diagonally press the Right analog-stick Up/Left to Down/Right to switch to the chase view. When you do this you will be able to see the rear of your vehicle and make sure the camera is not locked.
Disable Vehicle Collisions in Career ModeAdded 8 Oct 2010, ID #6374
At the 'Main' menu go to'Options' and then select 'Cheat Codes' and enter the case sensitive code 'ghost' (without the quotes) to be able to drive through opposing vehicles without taking of giving damage to either vehicle.
At the 'Main' menu go to 'Options' and use the bumpers to scroll to 'Cheat Codes'. Then press 'X' and enter the code 'SHOWTIME' (without the quotes) and ALL the locked features in the game will become available.