Hi @holl, sorry for such a delayed response. Here is a picture inside my retro kit. Not much to see. The red and yellow wires are coming from the 3 volt output on the step-down converter. You want to make sure you read the step-down schematic to get the + and - correctly attached. The red wire here is going to the positive terminal. @user36319 found a much more elegant solution as the battery eliminator goes into the battery compartment exactly like a real battery. It is quite a bit more expensive though. A bit too much for my budget.
