I’d expect that they’re on the same fuse as the fronts. If you can pull the switches out, check for power into the switch. If there is is power in, check for power out. I suspect it’s power in that is your problem, since neither are working. I don’t know if this circuit goes through a body control module after the switch or not. Maybe someone else can chime in.
I have seen on some cars that the back windows are blocked from operating if the top is not all the way down or all the way up. Not sure if the mustang works the same way, but maybe check that.