Hevy, I have to flip the latchplate in my Astro to get it stay when I install the Radian there. It's a little different, but it should solve your problem.
First you get it as tight as possible - just install it like you usually do. Then you stick your knee(s) on the carseat to keep it compressed, unbuckle the seat belt, and then flip the latchplate over, so it's "upside-down." Then you re-buckle it - and that's the tricky part! It's going to be really tight and you might think it won't reach. Just line up the latchplate and buckle as much as you can, then PUSH that carseat!
When I do it, it looks like this: PUUUUUUSH! Bounce! Push again! Swear! (The swearing helps, I'm sure of it!) *click* Ahhh.... :thumbsup: