[UPDATE] Turns out it was the buckle/latch that needed lubricating. Apparently when my daughter sat in the seat, the seatbelt pulled upward in way that rubbed the buckle against the pin that holds it in place.
Yes - thanks! You are right. It only happens with the seatbelt in use and the squeak comes from the latch area. It seems to only happen on the rear passenger side. I'll keep experimenting...
I am having the exact same problem! Back seat only squeaks when my 90-100 lb daughter sits in it. Did you have any luck in tracking down the problem? Thanks!