The locomotive is opened via the easily recognizable screw next to the slider from below:

The socket looks like 21mtc, but the specified space for the decoder is significantly too small. So some tinkering work was necessary.

After downloading the exploded view from the Märklin website, it becomes clear that the cab can be removed and the cover plate above the decoder can be taken off. This considerably increases the space for the CTC module.

Unfortunately, it’s still impossible to install it in such a way that the CTC module can be plugged into the locomotive board. So the decision was made to remove the locomotive board. Here is another picture of the wiring before I desoldered the wires from the locomotive board:

Still, the space is not enough to connect the CTC-locomotive module-21mtc with a plug. The CTC-locomotive module-21mtc just fits diagonally into the cab:

So I decided to solder the appropriate cables to the CTC-locomotive module-21mtc. The only remaining reason for using a CTC-locomotive module-21mtc now is that it is currently the smallest locomotive module from CTC. The cables for light, motor, and track are soldered on:

Soldering the wires for the tracks to the rectifier was not a good idea for this module: The rectifier was awkwardly placed by manufacturing, and the soldering caused a short circuit under the rectifier. So I had to desolder the rectifier and place a new one.

Then the power supply for the front LEDs and the support capacitor was still missing. After consulting with the customer, we decided to forgo the rear light: it was realized via LEDs on the locomotive board. Here is the fully soldered CTC-locomotive module-21mtc:

Locomotive module and capacitor (470 uF) found space in the cab:

And this is how the assembled locomotive looks:

With black cardstock, one could still hide the white WLAN antenna peeking out of the window on the left.