Try some BG Frigi-Fresh. There is a chance you got a sudden case of nasty mold growth.
http://www.bgprod.com/products/climatecontrol.html

"BG Frigi-Fresh® is formulated to quickly and effectively eliminate foul, musty odors from automobile air conditioning systems. It kills mold, mildew and other odor-producing organisms that grow in the evaporator core and housing. Powerful deodorizers in BG Frigi-Fresh® keep the automobile interior smelling fresh and clean. EPA Reg. No. 44446-20-67145.
Part No. 7083
Part No. 708"

Also, the blower is easy to remove in the W123 cars. You should pull that first. If it is a rat, that's likely the deepest it could get without crawling through a dash vent.