A few years back a member of the MBclubof America had this happen on a 1998 C230, he reported that the liquid is from the heating element and that when it got on his paint it left stains.

Go to a used Mercedes parts house for a replacement, I think removing the mirror from the housing is easy - it may be a snap or a release with a screwdriver do a search

BTW did you notice the mirror taking on a yellow tinge recently before the leaking.