Roco includes a detailed manual, which also clearly shows how to open the railcar: You just need to pull out the coupling hooks at both ends, then the housing can be lifted off upwards.

After removing the dummy plug from the NEM-652 connector, it looks like this:

There is currently no suitable CTC module for the existing NEM-652 interface, but with the help of appropriate adapters, the CTC locomotive modules for PluX22 and 21mtc can be used. Since I only had an adapter for 21mtc at the time of the upgrade, which could maintain the height of the installation space, I decided on the CTC-locomodule-21mtc. In the following image, we see all the components: The CTC-locomodule-21mtc with a suitable IR receiver set, the 21mtc adapter, and the NEM-652 plug with cables (only the support capacitor is missing from the picture):

I decided to place the IR receiver approximately in the middle. After removing the side-clipped bottom plate, I found a good spot for the IR receiver and a path for the cable. For this purpose, I drilled a 3mm hole in the bottom of the decoder installation space:

The cable comes out at the bottom between the wheel pick-up and the center of the housing:

Then the IR receiver is glued to the housing, the bottom cover is cut to fit and reinserted:

Finally, the IR receiver is plugged into the CTC module:

Then the NEM652 adapter cables are soldered to the 21mtc adapter board. One should concentrate well and preferably double-check, because even a wrongly soldered wire can permanently damage the CTC module or the locomotive electronics. In the picture, you can still see the solder point where I soldered the blue wire wrongly - the CTC-locomodule 21mtc received deadly 15 volts on a processor pin.

As almost always, this locomotive too lacks a sufficiently dimensioned support capacitor. There is no space for the usual 1000 uF capacitor, so a 330 uF has to suffice (left in the picture). A small plastic bar under the capacitor still had to be removed. In the picture, all cables are already neatly stowed, so the housing can then be closed again: