Flashed – Make Use of the Right to Refuse Testimony?

Hello everybody, I got caught today. I was flashed about 3 weeks ago (exceeding 23 km/h outside closed villages. Permissible speed was 80 km/h. Determined speed after tolerance deduction: 103km/h) Today came a letter from the state administration office with the heading “witness questionnaire”, payment request is not available. The car is registered to my mother, but I drove. It is required to provide information about the person, who My question, what happens if we, or my mother simply makes a crucifixion in the field “I make use of my witness refusal”? Can this become bitter? Will it then be investigated? Because theoretically they can’t prove it, my mother did not drive at the time. In advance already many thanks for your answers. and help =)