Go to the Towtruck with Tonya and then get inside. You'll get a report of an abandoned car somewhere, so drive there. When you get to the car, you'll need to lower the hook all the way down using the left analog stick. Then, you'll need to line up the front of the vehicle with the back of your Towtruck, backing up into the vehicle to make sure it's locked in place.
Once you do, raise the crane off the ground to lock the vehicle, then go and drop it off at the impound, inside of the designated area to finish the mission.