Pam,

Wolfgang International sells swing-out windows for the loaf busses. You can
have a bay rear hatch cut to fit your existing window rubber then install one
of these swing-out windows. Or get it cut-out of some pick-up truck then buy
a slider for it.

However, I would think than if you leave than rear open you will cause
exhaust fumes to be rammed in their when you drive. Translation: Exhausts
inside = dead driver.