Hi
The 37844 is an Fx digital locomotive released in 2002
The Fx decoder located in the tender is a one pice (no 21 MTC interface)
The motor is DCM (Drum Collector Motor) with a winded inductor.

All wires being black makes it more difficult to identify cables
Usually, if the platine/decoder are unwired, this is because it is faulty.
Märklin does not provide a wiring diagram for old Fx decoders intended for tenders (2002 version)

So what are the solutions?
ONE to replace the platine/decoder with a 60906 decoder for all current motors (your case)
Wire it like this

You will need to add a rear lamp holder (it was part of the old platine/decoder you removed.

TWO replace the platine/decoder with an mLD3 (60972) or mSD3 (platine version 60975, steam sound) that will fit into the tender
- you MUST replace the existing motor with a 60941 motor kit
- add a lamp or LED for the tender rear lights
- follow the colour schematics in these decoders


See the color codes in the red frame

Cheers
Jean

Hi
Usually, in Märklin cable colours Blue goes to the motor and brown is for chassis connection (so check if this is the case for you)
Here is my Märklin BR 50 of 2012 (same chassis but different decoder

Loco side


tender side


NOTE: there are differences between your loco and mine:
- in your case a wired inductor, in my case a magnet inductor
- in your case an Fx decoder, in my case an mSD sound decoder

Cheers
Jean