Let's start by the cab and end cars. There is a schema of how to open this car in the instructions sheet, but if it works at the first attempt it will be a miracle. First of all unscrew the bogies. Then you have to lift and push to the front the casing, but I found it was impossible to remove because of the windows. So I had to remove the windows to be able to open it. Closing it will be also a pain...
And opening it is the hardest work. For the light just cut and remove the two diodes. As it has bulb lights, you don't have to deal with the polarity like led.
In the bottom part of the board, solder the red and black wire to the current pick up strips. One of the pick up goes directly to the common of the lights. And white and yellow wires go to the other leg of the bulbs. In the following photos you can see the exact points where to solder every wire, you don't need to make any cut to the cooper paths.
And that's all the work, all wires of the decoder are soldered to the light board. Just mount it again. And there is room enough for the decoder. I used a LaisDCC decoder, it's a very small decoder and fits perfectly.
And that's finished, both directional lights depending on the direction of travel. Motor wires where cut, that makes impossible to read CV, but for this decoder is only needed to write the CV1 with the desired direction and nothing more.
The motor car is also very easy. Isolate with Kapton tape the motor leads, and solder in the right place the black and red wires (current), and grey and orange to the motor:
I hope it has been useful for you.