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