Okay, seat has been replaced. So here's the feedback.
My driver seat heater had two hot spots, one burnt through the leather quite bad.
Took it apart and inspected the element, yes I tore into it and it is a lot more complicated than I expected, and nothing like others videos on YouTube.

BMW seat is full of conductive carbon fibres that create the heat. They are incredibly this fibres, easily pliable, and tough to break. They are very good quality. These fibres are spread all over to produce even heating across the whole seat. I could not find a kink that would cause the hot spots, besides the fibres are very flexible and no way they would kink and stay kinked.

The replacement driver side seat cushions on the market are in heavily used state. So I bought a passenger side cushion. The leather is in a very good state. The passenger seat has an occupancy sensor and plug, otherwise it fit perfectly on the driver side. Installed it and simply tucked the occupancy sensor plug out of the way and all is good. Seat warmer is functional and leather is good.

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2004 X5 3.0