Hello,
That looks to be one of your crossover switches. Control the vales that control the EGR or Turbo.
You'll want to do the turbo wastegate swap.
Wastegate actuator swap for OM602.962?
Why you ask. Cause it will do away with all the stuff on the left side of the fire wall under and behind the air filter holder.
You'll not need to sort out the rats nest. It will cost about 100 buck but will be well worth it. Also you'll eliminate the EDS (computer from the equation) The computer will only control the idle rpm.
Speaking of idle rpm. Sounds like you're also having a problem there. The EDS controls the idle rpm. Some more info:
92 300D 2.5 won't idle on cold start,low idle speed on cold start