On planes like the Boeing 737 the rain repellent is applied in factory and is permanent, so you can't disengage it (inoperant switch). That's probably your situation.
Rain repellent avoids rain "sticking" on the windshield.
"A permanent rain repellent has been applied to the window at the factory. Each window wiper has its own switch.
Windshield scratching will occur if the windshield wipers are operated on a dry windshield.
1 Rain repellent switch (inop)
system deactivated"
Continental 737 aircraft operating manual, rain removal controls.
On the 744 manual I see that it's off, and that you can use it when you need it.
"note : do not use windshield wipers or rain repellent on dry window. If rain repellent inadvertently applied, do not use windshield wipers unless windshield washer activated first.
Windshield washer switch (as required) - ON
Rain repellent switch (as required) - Push - Use on one window at a time
Windshield wiper selector - As required"
Boeing 744 Supplementary Procedures - Adverse Weather.
Hope this made it more clear for you
