Bingo, Dawn is probably the best way to go. Almost every body shop uses dawn in the water they wetsand cars with. I've never heard of silicones in the soap but, dawn most definitely doesn't have any.
I just fill the sink of with warm water, mix in a bunch of dawn, dunk in the pieces and let them soak for a minute before I go to town on them with a toothbrush. Then I hit the body and chassis with 800 wetordry.
Oh did I mention I paint cars for a living? lol