Faulty sensor in seat

I bought a vw jetta 3 years back, they sold it to me with leather seats that they took to a plce to fit and vw gaurante these people. The passenger seat resently started not to pick up and warn you to put on safetybelt. When i took it to vw hatfield for service and told them about the probleme, they told me they are going to charge me if they open up. So i have to take it to the people who fitted the leather seats...now how am i suppose to know who these people are if vw sunnyside sold it to me like this and guaranty it? Emailed vw sa but no response from them either!