I'm retrofitting my vehicle with heated seats. I'm looking to use an LM338 to control the heating element, and I need a circuit that will allow me to vary the resistor value, adjusting the output voltage as a result. What I'm really after is a circuit that I can push a button and have it 'step' to the next resistor value. I've scoured the internet, and even attempted a few circuits myself, no luck. Anyone have any ideas?