Have any idea why MB SD connect C4 mux works only with 24V but no connection to 12V?

Customer solution… Here you go.

Try on your own risk!

some of the NEC Chips are Broken 4 PCs and same the small board power chip i Change it and now they work on 12V too

i change the first line from left side

Replace the chip

SD connect C4 china clone with boards of high quality:

Source: http://www.mbstartool.com/wholesale/mb-sd-connect-compact-4-star-diagnosis-cars-trucks.html

SD connect mux with PCBs above is confirmed to work no issues with Mercedes 12 and 24 volt cars.