Incorrect debit order, trying to fix

I have 2 cars financed recentlyn(Rio and Picanto)only 1 debit came off over the weekend and I called the call centre only to realise they had loaded my second car with an incorrect bank account nr. I have been trying for 3 days to get this rectified to no avail. My debit came off someone else's account. This person must be hysterical, as am I. I have mailed them repeatedly, called and been assured it is sorted and it isn't. I have still not been debited and nobody is responding. Mfc is known for loading people as late payers and I want to ensure this does not happen because of their error. So far the service has been badly lacking. I want this sorted today. The email box came back with a automated response asing for account number (which was on the mail). So clearly the mails are not being attended to. I don't want to make an EFT as I am concerned I end up with a double debit. I need someone to respons IN WRITING so that I have a paper trail and a contact person to deal with. The customer centre is not able to resolve.