Thanks for the write up! I just got a 2014 sienna a few months ago and was thinking about upgrading the back speakers, but keeping the stock HU (for now). As for back speakers, is there a max size of speakers I should be looking for to make sure they will fit? The Kickers you mention CS654, are discontinued now. So I didn't know if I go for the updated Kicker model (46CSC654) or the JVCs, CS-DR621 6.5" nizos mentions. I'm a newbie on replacing speakers. Just want better sound in the back so the kids stop complaining they can't hear, etc. Thanks!
Sure, glad it helped (or rather, that it may be helpful in the future)!
For the speakers, I think the important spec to check for fitment is the "top mount depth." I don't recall, or probably never knew, what depth is the max, but the discontinued 46CS654 that I used has a top mount depth of 1-13/16". The newer version of those speakers, the 46CSC654, have a top mounth depth of 1-11/16" so I would guess that it should fit too. It's a pretty cheap speaker like the older version and probably won't sound great but should at least be better than the stock ones. Other people have tried other speakers with success and you can find them in some of the above posts, although not sure which ones are still sold and available. I searched Crutchfield originally because they have such detailed info on speakers so there may be others that will fit too.
One thing that will make likely make a big difference also is installing some Dynamat behind the speakers, but that would require pulling off the side panels completely, and there is already limited space, but it may be worth considering when going through all the trouble in the first place. I plan to open it up later this summer just to install Dynamat.
Good luck and let us know how it turns out!