Welcome to the Fest. Yes, your problem could be caused by a faulty rear defroster relay. According to figure 3 on page 610-2 of the Bentley manual (you REALLY need to get one or download it), it is in the rear power distribution box. It is relay #6 which is small relay to the far right of the middle row.
It could also be the 15 amp fuse #28 (page 610-5). I would check this first as it is a visual check and cheaper to replace.
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