Okay, how do I get my "unlocked cars"?

[Mercer]

Am I missing something.. I'm sure it's obvious. I've just beaten #13, and I know I should have access to unlocked cars, but how do I get them? I can buy cars from main menu but how do I use them in my career? Can I? Thanks.

[Cops are cool]

You can't use cars from the my cars section in the main menu in career mode