Assuming the car is titled to the company, and the debt is company debt.
You should have a liability account set up with the starting balance = to the amount owed.
Post your payment using that liability account as the expense (reason) for the payment
when you get the next bill in, it will show you how much of the payment was interest on the loan for that payment. Then do a journal entry, back date it to the date of payment, debit interest expense and credit the liability account for the amount of interest paid.
make the new payment and then rinse and repeat each month
