On a rainy trip from Reno to western Oregon a couple years ago my wipers barely worked at all until we got into a deluge nearing the coast. Then after working better dring the tour they became recalcitrant again coming back. I had thought they were just being fickle, but suddenly occurred to me (duh) it was the altitude. My home in Reno is about 4500 ft elevation, western Oregon of course nearer to sea level. If your wipers try to work at all at higher elevation they might start working to a degree in North Idaho in the valleys (Couer d’ Alene about 2300 ft). Not a substitute for Rainex.