I have always used a slim jim in the area of the lock knobs. There is an arm which travels up and down in relation to how the knob operates. Therefore, you want to use your slim jim in an up and down motion. Roll your window down and shine a flashlight in the crack and you should see where you need to be. When the threaded rod is moving your getting close. Another thing to try is using the slim jim in the window channel to hold the threaded rod steady while screwing a new knob on. Hope this helps.

Andy