Our thanks go to Digitrax operator Aksel Suarez of Chesapeake, Virginia for sending this tip on how to install a Digitrax decoder in the Walthers GP15, called by some prototype railroaders a "Geepette" or "Cheep Geep".
Use a DH165A0 decoder to replace the existing light board.
Remove the plastic clips that hold the wires to the light board.
Once the light board has been removed, place a piece of electrical tape on the metal frame to isolate the decoder from the frame and prevent a short circuit.
The wires on the locomotive to the motor and lights do not follow standard wiring color practices.
To solve this issue, I printed out the Atlas equivalent decoder wiring diagram here.
Next was just a matter of matching the wire to the diagram and soldering it into place.
Because the two wires from the motor are the same color, my locomotive ran backwards with forward selected.
Changing CV29 to a value of 37 fixed that. (or you could swap the wires, a good reason for testing prior to closing up the locomotive, Ed.)