skylark
I'm Awesome
I use binder clips to grip the seat belt and keep it from retracting. I would be concerned that the sewing needle migh compromise the belt.I usually hang the carpet over my chain link fence and go at it with cleaner, a scrub brush, and then my power washer. The results are usually pretty good. While the seat belts are out, pull the belt completely out of the reel and stick a big sewing needle thru the belt to block it from retracting. Fill a bucket with water and something like Simple Green and soak the belts. I usually set the reel on a stool by the bucket and soak just the belt part. The buckle ends I just drop in whole. You'll be amazed at how much gunk is in there.
http://m.staples.com/Staples-Black-Metal-Binder-Clips/product_SS271759