The best example of a sunk cost related to the truck in this scenario would be: Money spent last month repairing a damaged front fender.
What is a sunk cost
Sunk costs are expenses that have already been incurred and cannot be recovered or altered by any future action. In this case, the money spent on repairing the damaged front fender is an expense that has already been paid, and its cost won't change regardless of any future decisions about the truck.
It represents a cost that has already occurred and is not relevant to future decisions about the truck, making it a sunk cost.