Brucie sends you emails about the vehicles that he wants. I don't remember how many there are, but after you complete the vehicles Brucie emails you about, there are no more vehicles that you can take to Brucie.
After completing the both Brucie's No.1 and Derrick McReary's Smackdown missions, Brucie will let you know that Stevie is interested in having you do some work for him. Stevie will then text you with the description and location of a vehicle he wants. After stealing and delivering the vehicle stated in the text message, he will send another message about a different vehicle he wants. All of Stevie's vehicles are to be delivered to the Garage icon in Bohan, just east of the save house you have there. After you have delivered all 30 vehicles for Stevie, you can take any vehicle you find to that garage and sell it. (not sure if you can sell police vehicles or not.)
Also note that Stevie will always wait 6 in game hours after the previous vehicle has been delivered to notify you of the next one.