Some cars have locked doors so you cannot steal them (such as cop cars and your first girlfriend's Hustler). However, getting them into a garage will unlock the doors. You can either push a car into the garage or get a Tow Truck or a Tractor to tow them. Additionally, to get in a locked vehicle with someone inside (for example, a Combine Harvester), shoot the person in the head and the door will fling open, allowing you drive it away. Find a Tow Truck (for example, at the dump in Mt. Chiliad) and drive to where the locked car is located. Hook the locked car onto the Tow Truck and tow it into your garage. Let the garage close. When you open the garage again, the locked car should be unlocked. This is easier then pushing it into your garage. If you try to open a car and it is locked, go far from it, then return. The color of the car should be different and the doors should be unlocked. If someone has locked their car doors, and you shoot them out, be careful not to get out before you reach your garage. Otherwise, the doors will stay locked.