What year is your JGC

I do remember reading a bit about some rather troublesome compatability issues with newer model years

Boosters aren't likely to help you anyway, though: not only are they not as safe as the harness, but it's a lot more difficult to buckle a
booster in tight squeezes compared to the harness which always stays right in front of the body

Also, what are the weights/heights of your 2 kiddos currently & do you have a budget just in case you do need to do some carseat swapping...?