Vibrating steering wheel on Focus

After a small front-to-rear low-speed accident in my 2002 Ford Focus Automatic the radiator was changed and, in another other workshop the timing belt was changed. I now have a vibration of the steering wheel hot or cold, and increases when I slowly turn the steering wheel to the right or to the left. This is very disconcerting and dealers and suspension specialists can't find anything wrong. On dealer agreed with me that I was abnormal, but could not fix it. Could you suggest any reason for this problem, or where to go to have the car examined?