I have trawled this forum and others with regard to this mod and, due to the mixed opinions, was wanting to see, through the use of a wiring diagram, the ins and outs of it all.
Technozen. I appreciate that you have extensive knowledge of all things MGR but surely that would mean that you too would have had to learn (and to some extent still do) at some stage and along the way would also have found out that occassionally someone will have a breakthrough in something like this.
Whilst to many it may seem like a futile exercise I would still like to explore this avenue.
You may get the LCD to power up but without the MEMS3 engine ecu where the data comes from you will have no odometer information, it will either read all 000000's or the last recorded mileage.
I too managed to get a set of ZR clocks working in a 200 several years ago, I even got the odometer to accept data from a PC, but as the vehicles recorded milage is kind of important I decided that 25/ZR clocks into a Mk3 200 is slightly pointless without this feature.
I even started work on designing the necessary electronic circuit to provide the correct data using the existing speedo transducer, but just couldn't be bothered.
You could replace the engine bay loom, new ECU, alternator, extra Lambda sensor, new inlet manifold.....etc.