I tried to reply before, but for some reason, it disappeared, so I'll try again.
Talk to a lawyer, but unless you have something in writing, you're probably screwed, as the courts always side with the service provider as long as they can justify the bill, and prove the work was done. That's why he is so eager to "Finish it". They have a right to keep your vehicle until they get paid, and if you don't pay within a reasonable amount of time, they can send for an abandoned title, and sell the car themselves, which may be their motive. They also have the right to charge a "Storage fee" sometimes as high as $100 per day, or more, while the car sits idle in their possession as you try to work things out. That can pile up pretty quickly as time is on their side. If your car sits there for a year while you guys fight it out, you could wind up owing them another $36k in storage fees, on top of what they are charging you. See if your lawyer can put a halt to any storage fees, as you are willing to take the car back, now.
It serves as a lesson to all of us to get a detailed written contract when you are going to have work done, even if it's being done by family, friends, or people you have dealt with before. The contract should include a provision where you have the right to remove the car if you are not satisfied with work (or lack of) at any time. It should exclude any "Storage charges" in the event of a problem. It should include a provision where no additonal work or "Unseen damages" may be addressed without your written approval, and if additional work is done, it should follow the same rules as the original contract. It should also include a provision that gives you the right to withhold payment from them, and use it to pay someone else to fix any damages they do or shoddy work that they perform. I know that this is hard to do when you are dealing with people you know, because we all want to be trusting, but I have seen money make enemies out of lifelong friends, and turn brother against brother. If you don't want to alienate them, tell them that you are borrowing the specific amount for the work, and the bank and insurance company insist on having the detailed contract to justify the amount of the loan, and to determine the future value of the vehicle when finished. If they won't do it, then go someplace else, or suggest a series of mini contracts, that each cover a specific task, or a set amount, like $2000-$5000 each.
You might want to consider taking out a personal loan to pay off the balance, just to get the car back into your possession. Or you can talk to a repo man and see if you can hire him to collect the car if it is left outside. Possession is always 9/10's of the law. Just make sure you put it someplace where another repo man can't find it.
Good luck.