And according to the ETM, the turn bulbs are not supervised by the external lamp monitor. At least not directly like the other bulbs. The turn signal flasher monitors the bulb current and if it doesn't have the load of 2 bulbs it flashes fast.

There is a tap to the monitor relay from the output of the flasher, maybe this is a special input to the monitor that looks for the fast flash pattern to turn on the warning light?

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech