To achieve this in Navigator, you can create a purchase ledger account for the vehicle - and post the payment from the finance company for the funding amount.
Then any interest payments you make are processed as purchase invoices which can be paid on the account.
Similarly any capital payments made can just be posted as receipts. Eventually this will net off as you will pay it all back when the vehicle is taken off funding.
If you need to include interest payments on individual vehicles the interest invoices could be posted as sundry costs to the vehicle to that supplier account.