How to complain to VW about VAG partners?

Hello everybody, I would like to complain to VW about a VW partner. Can someone tell me where I can do this? I bought a used car and found a defect at handover. The seller wanted to take care of it. After now 5 (!) attempts, incl. mail to the managing directors, you show no reaction even after several weeks. This is an absolute unding. Greetings Michael