99 Sierra driver's seat

I'm new here, but this appears to be a very informative forum. I just had a problem that really has me stumped. My driver's seat (bucket) with manual adjustment suddenly became dislodged and slid all the way forward on the tracks during a hard braking situation. Upon inspection I found a white plastic piece with a spring and a pin which operates with a lever to move the little pin back and forth lying in the right seat track. Now the seat won't lock into any position and to drive it I had to secure the seat with a cable attached to the back seat mounting bracket. The dealer looked at it and said they would order a new right seat track to the tune of $168. I feel that this is a safety issue and GM perhaps should pay to fix this even though it's not warranted, and I can't seem to find anyone else who has had the same problem. Any suggestions? How do I contact GM to get their people to look at this?