If you look under neath each seat, you will see something like a pack of cigarettes, with a wire harness going to it....this is usually the heated seat module......
Not easy diagnosis...a Tech II scan tool is needed......first each heated seat switch is part of the HVAC control module....when you press a button for hi or lo, this sends a Class II serial data signal to the BCM, Body Control Module, which in turn grounds the heated seat relay in the module, sending power to the heated seat elements......neither system has anything in common, outside of the heated seats fuse #13(20A), and the BCM.....
If both went at the SAME TIME(both modules/relays would not go at the same time), and it is not the fuse, then unfortunately, you could have a BCM problem......like I said, only a Tech II scan tool, will inform the tech, if the BCM is receiving a signal from the switches, and then If there is a Body code set(most scan tools and code readers can't read "B" codes)......
BCM's are expensive, and usually have to be programmed when installed.....so diagnosis is critical, that a BCM is not erroneously replaced....