Many cars are unlocked by completing events from the career mode. Go to your garage, select the division, and highlight the locked car you wish to unlock, it will inform you then what specific event you need to complete.
For example, in the GT division, the BMW M6 GT3 - Complete any Semi-Pro Touring Car Finale. Thus, you have to first unlock the semi-pro level (by completing a number of rookie events), then go to the Semi-pro Touring box, and complete the objectives of any events there that will unlock a finale, and then you will earn the car.
It can be as grindy or straightforward as you wish!