Hello @Sykes, and welcome to the pit 
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In your MIM-E03 controller (may be within the Indoor Unit if you have one, or a standalone box if you don’t), you’ll see the controller circuit board which contains a 6-terminal block labelled F1-F2-V1-V2-F3-F4:
- F1/F2 are used for serial communication with the Outdoor Unit
- V1 is a 12V DC supply
- V2 is digital ground
- F3/F4 are used for serial communication with the Remote Controller (MWR-WW10N)
Here’s a photo showing my (gold coloured) cable wired to F1/F2 which goes to my RS485-to-Ethernet adapter, in parallel with the white cable which goes to the Outdoor Unit F1/F2.
So as @toadhall says, everything you need for your MIM-B19N is here - comms to F1/F2, 12V supply to V1, and the earth to V2 (if you need this).