Asking here in the hope someone has come across this and may have an answer.
Car interior lights have an auto 'dimmer'. They do a graceful fade in and fade out when switched on and off. On a Merc. Owner wanted some slightly brighter lights so changed from tungsten to LED. The auto system still does a graceful fade out - but sort of leaps to on, rather than fading up. Later Mercs have got LED lighting which does fade in and out OK - but it's not just a case of changing the module which on the later cars is part of other things too.
On this model the module is stand alone, so would be possible to modify it.