Where do I go through customs from Phoenix, AZ with a 1.5 hr layover in IAD Washington DC, to Munich, Germany with United airlines?
When entering the Schengen Area in Munich, you will first go through Immigration and then Customs control. If Munich is your final flight destination, your luggage will also go through customs control there (otherwise at the final airport).
Within the United States, the flight(s) between Phoenix, AZ and Washington DC will be considered a domestic one. The United States generally has no exit Immigration or Customs control. Any sporadic checks that may be done would be in Washington DC.