Do you have a meter for checking system voltages?
Which seat is not working?
Is it just one heater element or multiple elements?
Does the light come on when the heater is turned on?
Please be patient and I will work with you to resolve this problem. We will be limited on the amount of diagnostics we can do if you do not have a meter.
Is the driver seat bottom AND back not working?
Ok, typically the heating elements are damaged from repeated use and the stress of being under your body. There are a few things you can try without a meter.
In the rear fuse box, swap the 10AMP HTDSLF fuse with the HTDSRF and see if the seat starts working
You can also try swapping the control modules from under the driver and passenger seat.
Check all connectors under the seat and be sure none have come loose. Also look for worn or broken wires from moving the seat back and forth.
If you can get access to a meter and/or test light we can check the circuits and narrow down the problem much easier.