FYI - PWM dimmers seem common in modern Jap vehicles.
I pulled a couple from 1995 & 1996 Hyundai Excels. They use a 10A switching MOSFET and appear to be PWM because there is negligible heat when dimming a 50W halogen bulb, and a parallel 12V LED dims in proportion.
I might find out how to reduce it lower dimming limit but otherwise probably won't bother deciphering the circuit - I think it's easier transplanting my old 555 circuit which uses ~10 fewer components, but the same 10k pot and FET switching.
It has a +12V & GND connection with the output being GND PWM dimming (ie, LEDs & lights to +12V and grounded through the dimmer's 3rd terminal).
I have little doubt similar circuits feature in other Jap vehicles...