Brian offers good advice, but if the dealer folds, it's just you and the borrower in court arguing over the validity of a contract that you can't prove was signed by the borrower. Adding to an already bad situation, if the court buys your argument that the signature is valid but then you can't prove the dealer had demonstrable consent to provide the TIL (and any other necessary federal disclosures) you're fully exposed to the Section 130a TILA liability. Plan for the worst and hope you're never in that position.
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...gone fishing.