Box 5a should be checked if their intent to keep and use the vehicle for their charitable work or improve it. You can deduct the fair market value for the vehicle in this case.
The Fair Market Value won't be found on the 1098-C however. This is something you have to provide yourself. Just in case, see the attached screenshot below where in TurboTax to enter this. Click to enlarge. It's on the screen immediately after the
Choose a category for this donation screen.
How do I determine the FMV of my car?
The most accurate method is to look for private party ads for similar vehicles for sale in your area. If you can't find sales ads for similar vehicles, the easiest way to find that is through Kelley Blue Book. https://www.kbb.com/whats-my-car-worth/