Take fuse out and inspect and clean it or put in new fuse.

Turn engine on... fan high and see if hitting the dash will trigger it on...

This happened to me and it was the brushes in the blower unit motor
that had worn. You cannot get replacements from merc dealer, new
motors range from £120 to £250. I made replacment brushes from some I
got from an auto electric shop. The blower motor is now running as
new. It cost me £2 for the brushes , a hacksaw blade and sandpaper to
to cut the brushes  to size and some solder. The motor is also quite
easy to get out. As an interim fix I lengthened the springs on the
brushes