I happened to have just remove the top anchor for my 3rd row seatbelt, and can confirm it is a 14 mm head.
Just to clarify for those following along, head size means very little. Bolts are generally measured by the size of the threaded portion. 11mm bolts may come in many size heads- most bolt sizes do. Cruiser Dan is referring to the size of the threaded portion when he quotes an 11mm size for this application.
-Spike