I'm sure this is a fairly stupid question but . . .

When / how do the headlight wipers come on? I never really thought about it but I noticed one of the wipers starting to fall apart on my 90 300E and bought a set. I thought they came on with the winshield wiper provided the headlights were on. Am I correct or is there a seperate switch for the headlight wipers?.

Mabye it's the back pain medicineI'm taking but I'm standing in the driveway like an idiot watching my headlight wipers not come on!

Thanks