I got the rear panel off and found (to my surprise) that the rear speakers are 5 1/4"! I guess I won't be installing them this weekend after all.
I ordered a pair of the MB Quart RCE-213's and will return one pair of the RCE-216's.
If you are thinking of attempting this yourself, be aware that the factory speakers are riveted in so they need to be drilled out with a small bit. I think it would be best to attach the replacements with a rivet too, so you'll need the right tools.
(This information is for the
Liftback - I don't know about the sedan.)