Is it the electric adjustment mechanism inside the mirror , or the hinge assembly that allows it to collapse against the window ?
If the latter , there are lots of aftermarket suppliers who do hinge assemblies on their own , as these commonly break if someone clips a door mirror and pulls it the wrong way . When I got my W140 , the elderly previous owner had caught caught the mirror somehow and it was hanging down looking sorry for itself ; on inspection the mirror itself was undamaged , but the hinge assembly , which was just a light alloy casting , was broken ( I think designed to be a weak link ) and I found loads of sellers selling these for around £10 a pair ; they were left and right handed . A bit of a fiddle to fit , but my mirror ended up good as new for a tenner .
If the electrical adjustment has died , possibly look on eBay for a complete used door mirror from any W204 , or look in a breakers yard ; they will likely all be the same internally , and as long as it is working , you can just swap over the exterior cover from your existing one so that it matches your car .
Notwithstanding yours is a C63 , I'm guessing that the door mirrors on even a C200 will essentially be the same mirror ? There must be plenty of W204s being broken for parts now . A scrappy won't charge you anything like £500 for a used door mirror .