Carrie,
I think my issue may be more severe. I took pictures, but my computer is giving me uploading issues. I have to wedge the Monterey tight against the wall. The booster pushes the belt forward so that the hard parts rotate down beside the soft part of the vehicle seat. Then, see where the stitching is on the belt? When I am facing the seat, I can see that the webbing travels to the left all the way to the top of the stitching, which is a couple inches, and then begins to go up and over the booster.
That sounds like a definite imcompatability to me. Couple of inches does not equal .5". So it sounds like that's another difference between the '03 and the gen including the '06... Can you do me a favor and measure from the anchor (where on the hard part the belt attaches) to the buckle itself, right exactly at the slot where the buckle goes in?
If you have other ideas, please do send them my way! This is our family vehicle, so it needs to be set up for fairly fluid every day use. I was trying to put two boosters in the third row, so it would be easy to fold down a seat when the older boys are at school or something, and keep the two harnessed kiddos in the second.
Truthfully at this point my only suggestions would be to do a three-across the second row and then put one child in the third. Between the third row dimensions causing problems, and the fact that your second row doesn't slide (right?) so you can't get a RFing seat in the third row, I'm running out of choices. I would suggest a ProSport, but if your 3rd row 40 is narrower than mine, there's no way it would fit. Mine barely fits, it's against the wall and wedged in there good.
Nisha's right, a FF Radian is do-able in that 40 portion but it is not easy. And for me, the recline angle ended up being pretty severe -- I believe I determined I wouldn't use it for daily transport. A FF CA might work, or something up on a base (MA, EFTA, MR.) If you can get the 5yo back there in that spot, things might be a lot more workable. My FFer who sits there now (4yo, sometimes a 5yo too) is able to buckle himself up totally, and I just have to pinch slack and/or tighten the harness up. Only if something gets misaligned to I have to climb up into the vehicle to get him straightened out.
My FFers just flip over the seat to get into their seats back there. With two three-acrosses, having a fold-down seat isn't an option. I really, really dreaded it when I had to turn my eldest RFer around (prior to this I had a RF 3-across back there and we loaded via the hatch) but it really isn't so bad. It helps that J thinks it's hilarious. It is harder with the older kids, though. (This is how my almost-10yo charge ended up sitting in the RF MR last week, trying to get out, lol.)
What about putting a combo seat in the back for the 5yo, putting the 3yo RF in the center, and then the two booster riders second row outboard? If you don't have LATCHable boosters, you could just pull one out to fold down the seat if you don't want him to flip over.
Putting the almost 8yo in a lbb could work in the third row - it sure does fit in there nicely. But I'm wondering if that is unwise because there are no airbags back there. I had thought to let him grow out of a hbb before transitioning to lbb.
I would prefer him to have outgrown the HBB, yes -- but I don't think it's *unsafe* to have him in a backless, at almost 8. You need to weigh what's best for your family with the added potential safety advantage of a HBB. I might try a second row three-across for now, and then transition him back there when/if you find it to be too difficult.
Now I'm wondering if I could put the 5yo's Radian in the third row on that wonky 40 seat. Add another hbb to that third row - either the Monterey or the Parkway if it ends up being okay to twist the buckle stalk. That would take care of the third row. ETA - Never mind! I've been out fighting hard with the RN, and I can't get it in that spot.
I did it once, but it was HARD. IIRC, recline the Radian, buckle, tighten as much as possible, recline the vehicle seatback, tighten some more, pull seatback upright. There may be more detailed directions in my post (link in my sig.) If your seatback doesn't recline, it may not be do-able. And as I said above, the angle was pretty crazy.
Then, the second row could have the 3.5yo in her Radian (either ff or rf - she's old enough where it would probably be okay to turn her, even though we were shooting for 4yo?). And maybe the 8yo could share the 60 part of the seat with her in the lbb. If he's in the middle spot maybe it wouldn't be such a safety issue?
I can fit RF RN and Monterey on the 60 in my second row. I think it's easier to buckle next to a RF seat than a FF one anyways. Beware that center seatbelt, I doubt that he will fit well there in the Literider. There's a number of issues with that position, lol. (Long buckle stalks and the height of the shoulder belt are the ones I think are likely to be a problem with your 8yo in the Literider.)
I'm happy to keep trying to work this through with you, I know how frustrating it can be to just want all of your kids to be able to ride safely (let alone conveniently) in this "8 passenger" vehicle. Once I got mine figured out, I really like it, but it's got its quirks and is a way different experience than a van.