This thread mentions a MAS relay. In fact, its pictured. My car has an open spot there and I always wondered what could go there. Now I know.
http://www.peachparts.com/shopforum/tech-help/266482-w126-relay-identification.html
But I am confused. The 5.6 may be different than the 300 the thread is about. It says: "
It seems You did not follow MB doc's explanation above , instead of a separate fuel pump relay and a separate KLima relay , the 1990 model has combined them together as a single MAS relay. which functions as a fuel pump relay + AC relay."
My 91 has a KILMA relay and there is no MAS relay. Just curious I guess.