The rear blower of your nissan quest is not working?

The most likely reason is a bad circuit in the heat / A/C dash control. There's a way to check it - there are 2 12g wires that plug into the control unit. These complete the ground circuit that goes to the rear A/C fan relay. There should be continuity from the control unit when the rear a/c is switched on.

By jumping the two wires, the rear A/C will work with the controller circuit disconnected, but "off" must be selected or the fan will continue to run with the engine off.