The EGR vacuum transducer is on the left side. The hose runs over to the manifold to operate the EGR valve. There is no direct measurement of EGR action (as in an electrical sensor on the valve), they measure the difference from the mass airflow sensor to determine if the EGR operated as commanded. If it does not see the expected change in flow then it throws the P0400.

I would check for vacuum leaks like you suspect. The climate dampers on the W210 are vacuum operated.

You can also try swapping the EGR vacuum transducer with the wastegate transducer on the other side temporarily to see if that’s your problem.

__________________
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