L.W.
Hi M.,
I would not recommend the seatbelt extenders! I bought one from the dealership where I bought my Trailblazer. They are not meant for car seats. We tried it and when you use that, the seat belt loses the function of tightening. When my oldest son was in his booster seat, I always made him buckle and then "ratchet" his seat belt so that it kept him more secure. Otherwise, he was constantly moving around looking out the back of the car window, etc.. with only the seat belt buckled. However, once we used the extender, he could not "ratchet" the belt. It was free to pull out as far as you wanted it to.
As far as a solution for you, sorry I can't help with that. I drive a Chevy Trailblazer EXT with the third row. Sounds great right...not! I have twins. When the twins were in their carriers or even their car seats now, big brother could not fit a booster in between. Also, with the split in the seats, I cannot get to the third row without removing a car seat. So, big brother gets to crawl over top the middle row to get to the back row. Major pain!!
I wish I could be of more help. But, all I can say is the extenders are NOT safe and are NOT meant for car seats. They are simply meant for larger people that cannot fit a regular seat belt around them. Good luck and I hope others have suggestions on what you can do.