Will faulty relay stop power to rear wipers,or only the intermittent function? Where is it located

page 12 has the wiring diagram for your rear wiper:

https://www.startekinfo.com/StarTek/outside/12265/disc_2/program/ETM/81_300_2.pdf

rear interval relay is located in the tailgate close to the wiper motor

from looking at the diagram I would say that a faulty interval relay could cause a non working wiper in all conditions.

disconnect the relay and by using a multimeter and looking at plug wire numbers from the diagram check your wiper switches and all

and of course dont forget the simple stuff like checking the fuse