Part of seat gets very hot with seat warmer - 2008 Ford Fusion 3.0

Most seat warmers have very small wires hat run in a grid. When voltage passes thru them they create heat. Like a Toaster

Very possible the wire broke.

Watch this video ,shows how to test .
CHEVY GMC SEAT HEATER TROUBLESHOOTING YouTube

You could pull the fuse or heating element connector till you fix the switch.

Easiest way would be to find the two largest wires and hook them up to a hot wire from the fuse panel. One would go to a good chassis ground, and the other to a switch you could reach easily, and then to the hot wire.

try under the seat to see if the wires not trapped in the runners and snapped?

2005 Ford 500 Limited can't start due to anti theft device, is there a reset I can hit

I would imagine that if you only felt the extreme heat in two specific spots on the seat, the element may have burned out and you basically have an open curcuit. According to the wiring scematic, the elements in the back rest and the seat itself are connected so neither work now. I've tried removing the leather from my seats to figure out if I can just replace the elements, but the elements are molded into the foam in the seat. Your only solution may be to replace the entire seat or to buy a ******** seat warmer. Most of these ******** heat warmers are rather cheaply made, though.

There should be plug for the heated seat most likley under it or on the drivers side of passenger seat. This should fix your problem, if it doesn't that means the short is in the dial in the dash. There should be a seperate fuse for each seat in the fuse box. If unplugging the seat doesn't help come back and i will figure out which fuse is the right one and where your fuse box is.

The most likely cause is the seat heater elements, they either have excessive resistance or are open - id recommended replacing both driver side element - upper and lower

the most probable answer to that is the valve connecting the heater to the seat has a hole or is not connected properly. Just to be safe check with a car mechanic