One of my kids dropped a dime in the back seat belt catch, then inserted the buckle. It became impossibly stuck, with the release button unable to be moved and the buckle and coin not retrievable -- tried brute force, lubrication, and all manner of thin tools. No luck.
The repair was not difficult. Turns out for this problem, here's what to do.
Source a Rear Seat Belt Catch -- mine was about $90 new online -- be sure to get the correct side; the right side has a single catch and the left side has two (for the center and left belts).
Pop off the seat cushion. To do this on a model with integrated booster seats, there are two 10mm bolts you have to remove first, and they are hidden under the booster seat instruction/warning label that you can see if the boosters are up. Peel the outside corner of that back, and you'll see 'em. Pop the seat up and out, and the seat belt catch is easy to see. One 14mm or 15mm bolt holds it in. Remove it, along with a wiring connector, and you can pull the catch out along with the stuck seat belt.
Now destroy the old seat belt catch with a hammer, being careful not to damage the belt buckle itself. This was the fun part.
You'll get it loose easily, and you'll get to keep the dime, offsetting part of the cost of the new part.Connect the connector, bolt the new catch back into place, put the seat back and bolt it in. Done in 30 minutes.