The classic sports car was bought by Homer in college, later replaced by the Simpsons' Family Sedan and the Canyonero. 

The Simpsons Hit & Run: The 70's Sports Car is unlocked in Level 6, Mission 7 (Kang and Kodos Strike Back), and can be used after in Level 7, and in any other level.  

Can be used by:
Homer Hit & Run
Marge Hit & Run
Apu Hit & Run
Lisa  Hit & Run
Bart Hit & Run