Inflatable travel pillow and suction cups or Velcro tape. There ya go. Dunno how a pillow stuck to glass would help you if you get rear ended. Do you mean in addition to a normal seat-mounted head rest or as an alternative?
Maybe the headrests you're looking for are unavailable these days for a reason?
BTW Why would you need to rest _while_ driving? You're supposed to be alert behind the wheel. If you need to rest, pull over. Be nice to know where you are doing this 'resting while driving' so I can avoid that area and keep my kids a little safer...
-- Dee
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