Rear-facing Wizard in Sienna--too wiggley?

Hello!
We just today bought a 2005 Toyota Sienna primarily because of its great safety features, but, after wrestling with my daughter's carseat in the parking lot of the dealership for about 20 minutes, I'm still not sure I have a good installation. My older child's forward-facing Wizard has a rock-solid installation, but the rear-facing one my 2-yr-old rides in still wiggles side-to-side. I have it installed with latch (also tried it with the seat-belts) and the rf tether attached to the base of the driver's seat's track. I think a lot of the "wiggle" just comes from the squishiness of the vehicle seat, but we have had the seat installed rf in the center of our Subaru Forester and it really didn't budge. Does it have to do with the fact that in the Sienna there is more space and the back of the seat isn't snugged up against the front seats? Also, I could tighten the tether more, but I think it makes the seat recline too far. The seat is in the recline position as per the instructions in the Wizard manual...should I try it in the more upright position? I will take it to a carseat inspection day, but in the meantime, does anyone have any tips?
Thanks very much!
Vannesa

I have a rfing wizard in the captain chair behind the driver in my 2004 Sienna. It is in with latch and it is as rock solid as the ffing wizard that has been installed in every other position in the van-lol! On the rfing seat I do have it tethered to the front seat, but it is not pulled tight because I like the angle of the seat the way it is. I do find that the lower anchors are pretty imbedded in the seat crease so it takes a bit of effort to pull the straps tight some times. It is also in the reclined position per the instructions.