Managing the loan or lease: Vehicle loan or lease

I am filing this formal complaint against BMO Harris Bank due to severe billing discrepancies, lack of account transparency, and a strong suspicion of predatory, unverified hidden fees at the end of my auto loan term. My car loan was established with a 72-month payment term. To date, I have successfully made 71 out of the 72 required payments. However, BMO is currently claiming that I still owe a remaining balance of approximately $4000.00. While I had previously utilized payment deferrals on this account, a remaining balance of $4000.00 at maturity is mathematically impossible under normal simple-interest accrual terms for a standard auto loan. BMO has failed to provide a transparent, high-information breakdown or an itemized accounting ledger explaining how this balloon balance was calculated. I suspect the bank is attempting to force me into paying unverified, hidden fees or has severely miscalculated the interest structure from my past deferred billing cycles. I was forced to threaten legal action just to be escalated to a Manager. When looking directly at the transaction history, the manager was unable to explain or calculate how the remaining $4000.00 balance was derived. They simply repeated that I had late fees and built up interest. I asked how that amounted to $4000.00, but they couldn't provide any mathematical connection. This proves the frontline staff couldn't verify their own accounting ledger, confirming my suspicions. I was put on hold for a long time with no explanation or any explanation of the connection.