Not familiar with Mercedes, but most seats have a separate plug that is the 12v. input, and then the other that does control. I suspect if you look at the door controls you'll see where a two wire plug is or was. Once you've determined where that is you can do as Dan said and temp power to it to get the seats moved to the position you like.
If you can't figure it out you can call a Mercedes dealer and talk to a line mechanic about where power is derived. As for controls, I'd definitely keep the electric controls, but I'd probably locate them in a custom built console and not in the doors of your rod.