Start by fully disassembling the locomotive and make a mark on the top of the motor.
The LEDs that come with the Walthers GP38-2 are standard yellow ones. I am going to upgrade to sunny bright white LEDs. I remove the stock LED's from the board and use them as a guide to bend the leads of the new ones to the correct length.
Next prepare the decoder. Cut the blue wire to a length of 3/4 inch. Strip about 1/8 inch insulation from the end and from one end of the piece of blue wire that was cut off. Twist the two stripped wire ends together and tin.
Now solder the two twisted together and tinned blue wires to the spot shown in this photo. Then route all the remaining wire ends through the hole to the top side of the PC board.
Next trim the yellow, blue and white wires to a length to reach the spots shown in this photo. Strip and tin the ends and solder the wire ends to those locations.
Cut the orange wire to a length of 1-1/4 inch from where it comes through the hole in the PC board and cut the gray wire to a length of 2 inches. Strip and tin the ends and solder to the brush caps after they are removed from the motor.
Re-install the brushes, springs, and brush caps in the motor. The gray wire goes to the bottom and the orange wire goes to the top of the motor. Use a small amount of Walthers Goo to secure the gray wire into the groove filed into the motor housing in the previous step. Then wrap Kapton tape all the way around the motor where the brush caps are to insulate them from the frame. Now reassemble the motor and worm gear assemblies into the right side frame as shown. With this type of mechanism that uses the rings to support the motor great care must be taken to insure that the rings are properly seated into the frame.
With the frame still laying on it's right side, place the washers that separate the frames into the holes and re-install the left frame. Make sure that the mounting rights are seated properly into both frames then re-install the screws and nuts.