I haven't figured out a way.
My kids are both in booster seats(using the standard seat belt) but my nephew is still in a full car seat (latch anchored, 5 point harness).
I have to pick him up every week or two to go between my sister and her ex and I feed the rear buckle under the head rest, it's thight but it will go and after the third or forth time I actually started to get pretty quick at it even though it is a PITA. I wish i could figure out how to raise the headrest to give myself more room to pass the strap through but I can't see anyway to do it. Other vehicles have had a little button, sometimes even so small that I've needed a center punch to push it bit I just can't find anything on the Terrain.