Being it’s date is in 2002 I’m assuming it’s a blue box unit. These are pretty easy to do once you’ve done a couple. Here’s what I do on blue box units.
Remove the shell from the chassis. On the BB SD40-2 the shell is held on by 4 lugs, 2 on each side. Simply spread the sides of the body and these will release.
Remove the contact strip from the top of the motor. This is the metal strip going from the motor to the trucks.
Carefully pull the motor up out of the motor mounts.
Turn the motor over. On the bottom are two metal tabs. These must be removed. I just break them off.
Place a strip of electrical tape in the bottom of the motor well between the motor mounts so the motor cannot make contact with the frame.
Carefully solder a thin wire (18 - 22 gauge) to the strip on the bottom of the motor.
Remember the strip that was on the top of the motor? Put it back on and measure 1 inch from where the clip snaps onto the motor and cut off the rest so that the strip sticks out 1 inch from each end of the top of the motor.
Cut a piece of wire long enough to reach from the contact arm of the front truck to the contact arm on the rear truck and make sure it has at least a couple of inches of slack. Solder one end to the front contact arm and the other end to the rear contact arm.
Put the motor back into the motor mounts and remount it into frame making sure the drive shafts go back together properly. Make sure the wire that was soldered to the bottom of the motor is sticking out a few inches so you can get to it easily.
Solder the red wire of the decoder to the contact arm of the front truck.
Solder the black wire of the decoder to the headlight clip.
Solder the gray wire of the decoder to wire that is soldered to the bottom of the motor.
Solder the orange wire of the decoder to the contact strip on the top of the motor.
For the lights I use 14 volt 25 mA light bulbs that I getr from Radio Shack. These have 2 wires, red and whit