I have a pre-owned 99 suburban in which the heated seats didn't work.There is a relay (module according to GM) that supplies power to the heating elements when the switch is placed in high or low.The ignition power has to be on,seat belt buckled and switch put on high or low setting.You should hear a slight click sound (relay energized) and the heating elements should warm up.If not, power may not be getting to the elements because of a faulty seat belt switch, high/low switch or relay.If these are fuctional,then the problem may well be the lower cushion heating element or the temperature switch for the element.On mine,I replaced the relay and put a jumper wire in the connector from the power input to the lower element to feed power to the upper cushion element.This worked to heat up the upper cushion element, but to repair the lower element, GM will sell me a new lower seat cushion complete for 450-500$$$.You could check with your local parts store and see if they have a supplier for just the element.I hope this helps.
The relay is partially buried under the very front of the lower seat cushion.
chuck's