Just in case the civil matter is weak, have you checked the state code? you have to do some digging but let’s just say i’m ‘familiar with’ a state code entitled ‘unlawful use of an animal, aircraft, vehicle or boat’ which sounds a little Arkansas/rednecky, but, it’s a class 6 felony, and very easy to make a conviction on thanks to the courts. We’ll say that I’ve been successful at leveraging the threat of this warrant being issued to ensure someone wasn’t screwed with for --hyptothetically-- having a firearm in their vehicle on private property against employment policy.
Your state may, or may not have a similar statute making it unlawful for someone to abscond with your vehicle. But wait you say, Mechanic’s lien! right, but you have video and presumably any “test driving” pursuant to an actual repair is far less than 400 miles and at some point the use of the vehicle transgresses from legitimate repair to, well... let’s call it what it is - larceny.
I’ve found in life that the possibility of losing one’s freedom and the ensuing stigma of a felony rap is a great motivator in making things right. Hopefully you do as well, OP.