The passenger seat heater worked fine up until a few weeks ago. Now sometimes it warms up and sometimes it doesn't. I have heard it is easy to break a wire by standing/kneeling on the seat, but to my knowledge no one has done that. My daughter does run the seat back and forth, seatback up and down, quite a bit when we travel. Could a plug have been pulled loose underneath? If possible, where is it? It's dark and there's not a lot of room under the seat to poke around.

 

Kneeling and standing on the seat usually damages the heating element, that will cause uneven heating, but should not cause intermittent problem. Looks more like a loose connection, or a faulty connector under the seat. You may able to see it under the seat it usually is tucked along with the sliding rail, if you try hard, but eventually, you will have to take the seat out to replace/re-secure it.