Yes, this problem gets posted often enough (mainly in the winter). Good thing you discovered it while it’s still blazing hot.?
This is a list of things you should check:
1) Fuses related to the seat heaters.
2) Loose connections at wiring harnesses under the seats.
3) If you know how to use a multimeter measure the resistance across the heater pads. There should be a finite resistance in the low ohm range; very high resistance indicates a break in the heating element.