What should I do if a seller refuses to fix hidden defects after buying a house in Sweden?

I recently purchased a property in Sweden, and after moving in I discovered several issues that were not visible during the inspection. The seller claims they are not responsible for these defects, but I believe they existed before the purchase. How are hidden defects usually handled under Swedish property law? What evidence should a buyer collect, and what steps should be taken before considering legal action? Has anyone experienced a similar situation and successfully resolved it?