If and when I do a seat heater, it will probably only be the driver's seat. I just came up with a circuit, based on a 7476 dual clocked TTL JK flip-flop, that would allow a single button (like my flap-ajar indicator, a modified blank plug for the button panel) to cycle through low (yellow indicator), high (orange indicator), and off (dark indicator), controlling one relay for power and a second relay to switch between low (assumed to be elements in series) and high (assumed to be elements in parallel).
Not going to worry about it until after I've taken care of my donut and my GPS navigator, though. Although I might build the circuit on a solderless breadboard, to see if it works in real life the way it does on simulator.io.
If there's any interest, I'll post a copy of the schematic. Although I can't say whether it will work as advertised until I actually build it with real components.
James H. H. Lampert