Since the van was listed as "like new," the listing constitutes a material misrepresentation and the law is on your side To write a demand letter, you would just set forth all the facts concerning the sale, stating the dates or approximation of dates relative to those facts, if you can. An example might be something like:
On January 1, 2016, I responded to the enclosed advertisement for the the 1999 Ford van bearing Vin number XXX X, which was purchased from you. At the time of the sale, you represented that the only thing this van needed was a battery, which I thereafter purchased and you installed. However, it turns out the electrical system is flawed and the van is useless without major repair. Since the vehicle is not as represented, you are in breach of the sales agreement and I am returning it to you with a demand for full refund of the purchase price. In the event you do not provide a full and prompt refund, I will file an action in small claims court and pursue all legal remedies available to me."
That's just an example. The idea is just to get the facts and your point across.
I hope this helps. If you need additional information or clarification, just let me know. I'm here to help. If I have addressed your issue, please remember to leave a positive rating when prompted.