On the bottom of the driver seat is the heated seat module. Look for the white connector with 5 wires. The orange/pink wire is the supply for the driver seat cushion, so that’s the one you want to switch off.
A couple issues though:
If the heated seat module detects an open circuit on either of the heat pad suppliers, it will store a fault and automatically shut the circuits off for the whole seat.
You’d have to use a resistor on a relay, so when you switch that circuit off, the heated seat module is still presented with the expected heat pad resistance.
Then, since there are three different settings, it’s looking for a different resistance for each setting, and will fault if the values don’t match. So basically, you have to choose which setting you want the seat back to be on when you switch the seat cushion off, and use a resistor of the same value of that particular setting.