I am not sure of what help I will be, as I am currently in the middle of installating my first sound decoder as well. I am putting in a Soundtraxx DSD-100LC into an Athearn dummy. I will post what I have done and maybe someone that really knows what they are doing can expound a little, and help fix any errors that I have made.
I wanted to run the least amount of wires between the two units so I decided that I would just pull power from the dummy. Besides the fact that I needed to replace the plastic wheels with metal ones, I also found that the dummy has two left hand trucks. (at least mine did) With the plastic wheels it was not a problem, but as soon as I replaced them with metal wheels the chassis would short out. I found that I needed to hard wire the pickups, and try to insulate one of the trucks from the frame.
The first truck I just hard wired the right hand side pickup (Red) and just left the left hand side the way it would normally transfer power through the chassis. The second truck however, I had to hard wire both connections and insulate the left hand side pickup by placing electrical tape over the plate that connects to the chassis.
