What usually goes bad with the seat heaters?

NeIther seat heater works in my new-to-me 04 Explorer limited. Pushing the buttons does light the indicators on my climate control display -- but that's all that happens.

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Normally a wire will burn out on the element itself. I repaired a seat out of a 2001 limited this way to install on my 93. You have to completely dismantle the seat to get at the element which is not too bad just time consuming. Then splice the wire or put in a new element (ebay i think has them).

All you need to know about dismantling a seat:

The Seat Shop Install videos

The heating elements themselves can and do break. They are subject to bending and twisting forces as we enter and exit our vehicles. The failure point will most likely to found
at or near a sharp bend. Replacing the elements is pretty simple.

One final point - if the elements were taped to the seat cover or the foam of the seat (with a thin metal reflector/heatsink that must be reused lest the element literally burn out while your butt is sitting in that seat and that is not fun for anyone.