The '59 Cadillac hearse was built by several manufacturers(S&S, Miller-Meteor, etc...) and all were offered with either forward, or suicide (3-way, side loader) doors, and the rear windows were covered on both models.
I have converted many of these bodies David, it's not too hard to do. Just take a piece of card stock and hold it on the inside of the body, and use a pencil to draw the shape of the window, and cut it out about 1/8th of an inch outside the line you drew. Transfer that to sheet plastic, and reverse it for the other window. Good luck with your build!
I think I'm going to build mine with glass all the way around. I plan on building a roller deck and casket for the cargo are and I'd like for all of that to be seen.
Also, I noticed in the picture of the wagon you're building, there are cut-outs on the right quarter panel. Mine has the cut-outs on the left quarter panel. What kit is yours from?