If the black and blue wires to the trucks match, they are to the same side. Use the decoder's NMRA standard wiring color code to connect it -- black to the left wheels (decoder's black wire); blue to the right wheels (decoder's red wire); decoder's white and blue wires to the headlight; decoder's orange and gray wires to the motor.
Don't cut the wires -- unsolder them from the terminals on the motor.
Get yourself a VOM (Volt-Ohm-Milliammeter) from Harbor Freight ($2.99) or similar source and learn to do some simple wire tracing. A VOM will tell you, for example, which wheels the black wire is connected to and which have the blue wire.
Trying to bring this Walthers Trainline locomotive up to DCC specs. Have read a lot about such things, but amazingly still confused. Thought I'd ask before cutting any wires.
There is a black and blue wire going from the motor to the front and back trucks, then a red and black going to the light. So it isn't too complicated looking, but hey, I'm new at this and trying to learn!!
The decoder is a basic Digitrax one, and yes, it comes with a diagram but they only show wires going to one set of trucks, not both.
Any advice?