Was it purely a business vehicle or also a personal vehicle? If it's a personal vehicle, you can't deduct any loss, you just need to figure out if you have depreciation to recover. You first need to know the amount of depreciation you claimed, over however many years you used the car in business. The depreciation was claimed as a vehicle expense on your schedule Cs, or if you used the standard mileage rate, part of the mileage rate is counted as depreciation (exactly how much is given in a table in publication 463.)
https://www.irs.gov/forms-pubs/about-publication-463
Pub 463 also has the rules for calculating recapture when a personal car partly used for business is sold.
