What to do if a car repair service company destroyed my car?

If you believe they damaged or destroyed your car, you can submit a claim to them for the value of the car and other out-of-pocket costs you have incurred (e.g. renting replacement vehicle). They may or may not take the claim seriously; they may or may not submit to their insurer; their insurer may or may not investigate; if they investigate, they may or many not pay. They would not be required to share the results of their investigation with you. In short, while there is no harm in submitting a claim, you can't make them act on it--for example, you can't make their insurance "determine root cause" for you.

The only way to seek compensation if the service company or its insurer will not voluntarily pay you would be to sue and prove in court that they damaged or destroyed your car. You would have the ability to get testimony and records from them, but would also have to provide your own evidence. You would only win if you can prove your case by a "preponderance of the evidence," or show that it is more likely than not that they caused the damage. Since winning is not a given, and the amount you can recover is commensurate with your vehicle's value, for an older vehicle, it may not be worthwhile.