just to be clear I have never done or witnessed a turbo conversion in person. Im just trying to help with the information infront of me.
So it looks like the 4 way coupe connector is the
'149D Front Cable/Fuel injection cable connection' you can buy these AMP connectors and pins new easily I have made them up before.
HV 141 - Grey / Green
N 29 - Black
HL 58 - Grey / Blue
HR 57 - Grey / red
I found a page in the coupe manual showing two of the pins are part of the CODE system.
Wire HV (Grey / Green) in connector 149D
goes from;
pin 6 on part 284 The code box
to;
pin 47 on part 141 the engine ECU
Wire N (Black) in connector 149D
goes from;
Pin 4 on part 284 The code box
to;
Common Ground
This page explains the part numbering and locations;
The next two pins are on even more blured pages :S Hope I got these 100% correct.
Wire HR (Grey/Red) in connector 149D
Goes from;
Part 57 - Oil Pressure sensor
to;
Oil Pressure guage on Dash
Wire HL (Grey / blue) in connector 149D
Goes from;
Part 58 - Engine Oil Temperature Sensor
to;
Engine Oil Temperature Guage on Dash
Have you traced through all the pins on both the 20pin Bravo and Coupe connector? there may be differences.
Hope this helps. I will try and confirm what the 2 way Bravo connector goes to. (probably code box as you alread said)
I'm converting a 147 hgt with a cabled throttle. It's not the lambda plug as lambda is plug in. I've drawn a little detail on a photo. Also a close up photo of the wiring colours on the 4pin plug. They appear to be BLACK, GREY/GREEN, GREY/BLUE AND GREY/RED going on the colour chart.