I had the same problem in my 2001 SSEi. Pulled the seat, removed the leather cover and the foam, and found the electrical feed wires are soldered to the heat element wiring at three locations. At two of the three locations there was a broken wire. I cut into the foam to expose a little more wire, resoldered the connection, buried the connection in a puddle of epoxy (if you take the seat apart you will see what I mean) and put it back together. Works fine. For now.
The threads in this forum recommend you purchase an after market set of heater elements, and install them on top of the old elements (between the white foam that is sewn to the back of the leather cover material and the yellow foam that is the seat cushion). That would certainly work, but like I said, I was able to repair mine.
How good are you with repairs? There is a small green connector under the seat that feeds power to the seat cushion, and another one between the cushion and the seat back. If you disconnect these two connectors and use an electric meter to check resistance, you can determine if the problem is a broken wire in the elements. I think the cushion zone should read around 3.2 ohms, and the seat back 2.4 ohms, but I am not completely sure.
If you decide to do this, make sure you know how to deactivate your seat airbag system first. don't be playing with the connectors under the seat without doing that.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)