C180 electric windows button has to be kept up to close window

If you're referring to a switch which is usually has a "one touch" operation, then it's a partial failure of the electrical relay for that window.

To confirm, if the passenger window also has one touch then swap the relays over (the vehicle handbook will tell you where they're located). If it's the relay then the problem will be swapped from one side to the other. If not, then it's the switch.

Unless you have auto-electrical experience then get this job done by an auto electrician, or by your Mercedes dealer if you want to pay through the nose... It's usually a quick and (by Mercedes standards) a cheap fix, but if the switch is at fault it may cost a bit more.