Has anyone changed the front/rear interior bulbs to brighter ones that still fade?
I would expect the dimming to be realized by PWM (pulse width modulation) ie switching power fully on/off very rapidly, slowly increasing the off-time and decreasing the on-time to dim the bulb - so pretty much any regular bulb (without internal driver circuitry), LED or not, should be able to dim.
The problem dimming LED bulbs on the mains usually comes from the switch-mode driver in the bulb that takes the 230V and chops it into whatever voltage is required by the LEDs. If you dim the supply, the internal driver will just 'work harder', still trying to run the LED at full brightness, until the feed is low enough that it simply no longer can...