Product: Vehicle loan or lease

GM Financial Account XXXX - GM Financial has dropped my credit score from over XXXX to XXXX, and is not even hearing me out - I have submitted a lengthy report with supporting documents to them through the XXXX  - I am now at the end of the lease and my credit is bad and it is all because of them - They tacked on a fee which was never communicated to me, and was never explained - I tried to pay for it, correct it, dispute it and simply just ask what it was for - After 3 years of phone calls, emails, website messages, and paying extra, they did not help - All I wanted was to get current on payments, ( So I could set it and forget it with auto payments ) - You cant set up auto pay with them until you are current - This was difficult, when the fees and calls for fees with no explanations, keep coming in - I have a detailed list of all the employees, employee IDs, dates, and correspondences - I have the Account Payment History they sent me, my hand written log, my bank records to match it - I have everything you could possibly ask for - What I still dont have is the original reason/date/amount, that started this late payment or fee - None of this would have happened in the first place without this first fee discrepancy - This unexplained, non-communicated fee or charge is the catalyst for my entire payment history with them Reasons GM said I might owe : - System Issues - Missed payment possibly ( XX/XX/XXXX ) - Local taxes - 32-Day late fee - Missed payment possibly ( XX/XX/XXXX ) - Missed payment sometime between ( XXXX XXXX ) To this day, all they have to say in response is a letter that ( STILL HAS THE WRONG ADDRESS ON IT! ) : Upon review of the account payment history, our records reflects the payments in question for XXXX, XXXX, and XX/XX/XXXX have been applied to the account. ( XX/XX/XXXX ) This does not explain anything. 
It does not explain the original fee that started the problem In fact, it does not even explain any missing payments at all! 
It simply says all payments have been applied This is not even why I contacted the XXXX on them! 
This letter they sent ( I have attached ) is so bad, it didnt even fix one of the possible problems of my address in the top corner. And if that isnt bad enough, they sent the same response letter with the incorrect address twice! 
My complaint is that they ruined my credit with no explanation. 
I DID find a missing payment, but that was way after the fact, and because they would not let me get current so I could use auto pay. Furthermore they didnt even find that missing payment, I did! And in good spirit I admitted it during my research for my XXXX  response. They didnt even find or communicate that one to me. 
Summary- Sometime in the beginning of the lease they charged me a fee and didnt tell me. 
When I caught it a little bit later, they couldnt explain, it and told me to ignore it. 
This is when the many reasons started coming in After some waiting, I began faxing them proof of payments, emailing them, calling them, being told to wait again I paid late fees, and anything I could to try and end it. 
Recently I reached out to the XXXX. No Luck there. 
So now I am in need of a new car, and my credit is making it too expensive to get one. 
I have attached all of the XXXX documents, with dates and references.