I've got the same issue on my 2007 Coupe. But I'll troubleshoot the failure if it ever cools off!

This link to wiper & washer system for a 2004 M-B SLK 320 will also give you part numbers that work (not everything, but most): CLICK. The Crossfire washer reservoir isn't heated.

The attached Forum DIY document probably will help in both our efforts.