I am looking for a YA book about a girl that has a horse with a mysterious silver buckle or medal that she puts on her horse. When she touches the buckle, she can make a wish. One of the wishes she makes causes a school bus crash. The buckle is linked somehow to a dark force that ends up appearing at the end of the book. It’s a pretty dark book. I’m thinking late 1980’s or very early 1990’s.
This has GOT to be VOICES IN THE DARK by James Haynes (1982). It’s #6 in Dell’s paperback TWILIGHT: WHERE DARKNESS BEGINS series. Here’s a bit more information. https://series-books.blogspot.com/2016/10/twilight-darkness-4-fatal-attraction-5.html
VOICES IN THE DARK (1981) by James Haynes.