That is the connector to the vehicle's Constant Control Relay Module or CCRM. Ford used CCRMs on many car lines begining in the late 80s and early 90s. Essentially it is a electrical module containing several relays that are controlled by the PCM (computer) to turn on/off certain things like the electric fan or AC clutch compressor. Mustangs did not use a CCRM until 94. Not being familiar with the 94-95 Mustang harness I cannot tell you whether you will need any of the CCRM functions in you Miata conversion or not. Schematically some of its wiring is shown here:
http://www.veryuseful.com/mustang/tech/engine/images/Mustang-94-95-CCRM-AC-Diagram.gif
There are more 94-95 schematics here:
http://www.veryuseful.com/mustang/tech/engine#95-95Diagrams
I read your other post and I doubt you will find a physical diagram of your 94-95 harness- Iver never seen one.
I only put EFI 5.0Ls into old Fairmonts - your project sounds awesome.
Good luck guys
T