I got curious after reading your question. Found this in the owner's DVD:
For rearward facing infant seats secured in the center seat position with the vehicle seat belts, the rear center seat position has an armrest tether that secures the arm rest in the upward position. To access the center seat arm rest tether first lower the arm rest. The tether is located behind the armrest and hooked onto the plastic seat backing.
Pull down on the tether to unhook it from the plastic seat backing, then raise the armrest and attach the tether hook to the strap located on the front of the arm rest.
There are pictures of this also.
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