If your ignition key will not return to its proper position, the problem is not with the lock. There is a small module attached to the back of the ignition barrell where the ignition wires connect to. This module is spring loaded and is supposed to return the key to the "on" position after the car is started. Often, the spring return does not work properly. The module is held on by one small screw.

Just find a second hand one at the wreckers and replace it.