Old VWs (Cabriolet, Golf/Rabbit Mk1 for sure) have dimmers for interior lights that are essentially a 20ish Ω Rheostat connected to 12V when headlights switch is turned on. Rheostat solution works fine until there are at least few bulbs (in switches, instrument cluster backlight, ashtray light, blah, blah, blah). If you replace all bubs with LEDs, rheostat just won't be able to work as a dimmer. This circuit connects to that rheostat and few other places in car (12V, GND) and outputs 0-100% of input voltage depending on rheostat position. Circuit itself is nothing more than buck that has analog voltage from DAC injected into feedback node. The whole shabang is controlled by attiny13a.