I have no idea how GST/HST comes into play since I am in the US
book the initial 1K as a deposit to the car dealer, use a current asset account you create called truck deposit
create the fixed asset, and liability accounts for the truck and the truck loan, zero starting balances
On delivery,
journal entry, debit fixed asset 1K, credit truck deposit asset 1K
Write the check for the additional 5k down payment and use the truck fixed asset account as the expense for the check
open the register for the loan liability account, make a new entry as an increase in the amount of the loan, use the fixed asset truck account in the account block
That is the accounting entries, how you deal with gst/hst is something your tax accountant should be able to explain I hope.
Cost is what you pay, the rebate is really just their promotional discount
