First of all, I have to thank the help posted for this digitalization to the Agenz people (see their article), and all warnings from Quinntopia after he burned at least one of this (see the posts). Thanks to these people my digitalization went straight forward.
My first advice is to use a really tiny decoder. I first tried Digitrax DN135D, with measures 13,86mm * 10,3mm * 4,83mm, but it was still too big to fit in the weight hole. Finally, the only suitable decoder that I found was a Digitrax DZ125 (10,6mm * 8,7mm * 2,86mm). Even with that small size, I had to trim the protector to fit it in.
Step 1: dismount the cleaning car starting by removing the screw of each bogie. You can also pull the rounded plastic part in the bottom that acts as a vent, but I think is safer to do it later polling from the motor. With the screws removed, lift the plastic covers and the weight inside. Last, pull from the motor and remove the circuit board:
Step 2: remove all electronic components from the board and discard the plastic piece that goes in the hole of the board and it's connected to the switch of the top cover used to change the motor speed in analog mode. Once the board is clean, it's also recommended to cut it a little bit to help decoder to fit inside. Be careful you don't cut it too much (just a pair of millimeters) or you will loose the part where it takes the current from the wheels.
Step 3: Prepare the decoder cutting the wires and tin them (see the right position in photos below). I recommend also to use a good "wire trimmer" (sorry, I don't know the exact name of this tool in English...) because decoder wires are really weak.
Also apply tin in the following points were you'll solder the motor and power wires. I use very thin tin, much less than 1 mm (I think this is 0.2 mm), or the tin drops will be too big:
Step 4: Solder the decoder!! In the photo below you can see the exact position and where each wire goes. Of course you can cut the function output wires, but I want to keep them to try to put a flashing light or similar:
Recommended: Isolate the board as you see in the photo. I think it's not really needed, because current tracks of the board don't touch the metallic weights (those metallic weights are powered from the tracks), but it's always worth being prudent:
Step 5: And now you can mount everything again. The only pieces to discard are the switch of the board (I mentioned before) and you can also remove the top case selector, but do it carefully because this part of the hull is where the screw is fixed.
I removed the selector because I wanted a blinking light when the cleaning car is active and running in the forward direction. Yes, not beautiful... but I hadn't at this moment SMD leds nor resistors, so it will do the job. Finally, it's just a cleaning car, not a prototype, and inside there isn't room for anything else I guess....
I hope this guide helps you to avoid buying special NEM boards for this car that in my opinion don't help to fit a decoder inside, just the opposite!!