This entire situation is a little strange. Unfortunately, there is not likely a quick remedy here; instead you may have to institute a legal action to get the seller to turn over the vehicle.
I would suggest that you consult with a local attorney immediately. I can offer a free consultation in Wisconsin Rapids.
In accordance with the Avvo community guidelines, this communication does not constitute "legal advice", nor does it form an attorney-client relationship. You should seek counsel in your geographic area regarding any specific questions.