Re: 2012 GMC Yukon XL latch.
The new LATCH manual isn't coming out until early spring.
I would check the vehicle manual for hints.
Sometimes vehicles allow borrowing an anchor from each outboard position if the car seat allows it. But then you can't use the anchors for outboard seats. It may be a catch-22 for the anchor positions.
Rear tethers may be located in the ceiling or some place up rather than down ;-)
I'm mobile at the moment, but if I remember to check the LATCH manual for earlier models, I'll be sure to post.