This is an easy one:
You draft a demand letter to both the dealer and mechanic on the same letterhead and inform them both that they fail to refund your money by a specific date, you will seek relief from court via filing a small claims court complaint.
Gather all supporting evidence to prepare for your trial date such as witnesses and documents to present to the judge.
You will win this one easily. They can't expect you to pay for something that was defective.
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