* The vanity-lights in the sun-visors shouldn't "short out" just from being "left open". (It might drain the battery, or shorten the life of the bulb... but simply being 'open' wouldn't/shouldn't cause damage.) The actual failure may have been due to lifetime wear & tear, or perhaps even atypical use prior to the rental period... in which case FlightCar's responsibility is murky.
* The VW dealer price for repair is insane. Yet even though customer is displeased at FlightCar having slightly overestimated the radiator/themostat fix cost (by $200), customer expects FlightCar to cheerfully accept highest-possible cost for his own reimbursement.
Doing what this customer seems to want – generous reimbursement for something that isn't indisputably related to the incremental use during the rental period – might not be tenable for the business even if everyone was acting in good faith. And if FlightCar had a reputation for generous reimbursements, it would also attract people acting in bad faith: blaming FlightCar for preexisting issues that could be snuck by the check-in/check-out inspections.
The difficulty of tracking and resolving such issues may be a fatal flaw in the FlightCar business model.