Did you get the Solobaric L5's or L7's?

The way I see it (my opinion), if you have a good set of speakers up front, and even a sub or subs in the rear for bass fill, there is no need to rear mids. You'll throw off your sound staging and it probably won't sound as good.

That being said, I know Infinity makes a 4" that will fit in the stock mounting bracket. I do NOT know how they sound as, I personally have not heard a 4" back there since I replaced them. I use to have a 6.5" and 6x9" Infinity Reference back there, and it sounded decent... but I do not have any speakers mounted in the rear mounts now.