I've had this type of scenario with electrical accessories on several cars. This includes central locks, lighting, washer/wipers, seat motors, etc. I have not been able to figure out why. When the item stops working altogether and won't come back then I deal with it. Many electrical connectors, splices, etc. are subject to uncertainties due to temperature, corrosion, etc. One can be certain there is a logical explanation but it can be a bear to find.