Speaker sizes in a nutshell -

86-87 - 4x6 in dash, 6.5 in rear with room for separates

88-91 - 4x6 in dash, 6.5 in doors with room for separates, 6.5 in rear with room for separates

My uncle had an 84 500SEL in which 6x9 subs were fit into the two cubbies in the rear deck.

Sixto
95 S420
87 300SDL