SOLVED. Middle grades/juvenile mystery series; teen detectives drove a hearse. [s]

[Mai]

Maybe The Mystery of the Marble Angel by John F. Rambeau? I looked it up online and people mention the purple illustration. It's #2 in the Morgan Bay Mysteries series. It's about two teenage brothers and their sister. In this book in particular a hearse is mentioned:
Here's a story of three teenagers, trying to buy a car - an old hearse - to have as their own with the idea of using the back to store their surfboards. A fun story told in the early 1960s gives shades of the tone of a Perry Mason as they become embroiled in a mystery that involves their being accused of a crime they didn't commit - to cover a bigger crime.

(it says three teenagers because their friend is the one that first gets the car, I think)