To unlock the garage level from the start of Driver 1, follow the instructions below:
First, you need to complete Smash Activity I. This unlocks you the DeLorean (yes, from Back to the Future). You may be able to see where this is going.
Go to a garage and purchase the DeLorean. Take it out onto the road and reach a speed of 88mph. A message will then pop-up telling you that you have unlocked the 'Blast from Past' challenge - the Garage level. Enjoy!