How can I get Stevie's garage to open after story is complete in GTA 4?

Did you steal and deliver all the cars that Stevie asks you for via text message? You've got to do all those before you can deliver just any car to him. Also, I think he's got 'standards'. You can try stealing one of the cars from the high-end auto dealer in Algonquin, he's got to like one of those...