Who should I contact to get my car repaired dealer, warranty, or insurance ?

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I purchased 2017 Buick Cascada nov25, 2019 I noticed it had a leak in conv. Top Dec14,2020.id like to know how long can the dealer Carmax have my car when the part is on backlog. And when should I expect my car back. Carmax didn't give a date or time. I purchased extended warranty and have full coverage the car was paid off Dec17,2020. Looking for suggestions on how to approach this issue.