Mazda provides seat belt extenders for the front seat but not the rear, and the belts are incompatible. If you need a seat belt extender for the rear, you can buy one from any number of retailers or eBay, you just need to find a "Type B" belt.

See attached picture- it shows a car seat with an 8" RIGID type B seat belt extender. It's been a life saver for my fingers, and I feel more comfortable safety wise having a seatbelt my kid can buckle and unbuckle himself. You can find them at various online stores or on eBay- I paid $20 or $25 for this one, shipped.

I first tried a 3" extender, but it was not long enough (that's what she said). I think a rigid one is the way to go though.

The 8" extender does not seem to adversely alter the way the seatbelt lays across my child's chest or lap, I'm pretty happy with how it's working out.

I noticed not all sellers claim to have DOT approved buckles, so I'd probably be aware of that when looking for one.

 

Also, I've tried a couple different booster seats- I had the GRACO Turbo Burster but the base was so wide, it didn't fit correctly. I went to Target and turned all the car seats over- I bought one with feet that are inset from the sides a bit (see pic). It fits perfectly!

 

Things get a LOT better when the kid turns 4-5 and is big enough he can sit in a regular booster seat which is nothing more than a padded plastic hump with 2 armrests on it. I did have to put some stuff under my kid's booster to shim it up some so that he wasn't sunk way down in the back seat, though.

 

This is actually is a booster seat for my 6 year old. ;-) A lot of 4-5 year olds I've seen still benefit from a seat with a back because it helps adjust the shoulder belt to be at the correct height.

 

For real, your kid is 6 and he sits in that? Seems rather bulky. I looked at the full-back booster seats but couldnt justify the extra 75 bucks they cost and the bulk...it is so easy to transfer the small base versions from car to car with no significant "positioning"...in fact he can grab it and move it himself if we ride in a different vehicle.

Maybe my kid is just big for his age...he would have been quite stable in a standard "hump" booster seat at 3-3.5. He's 5 and is the same size as most 6-7 year olds in his class.

 

Ya, as long as the shoulder strap fits well, the back part isn't needed- it all depends on the size of the child (and your state child seat laws).