You only need to get the plugs through the holes.
Use extra long cables and route them all out the top of the seat tube, plug them in, then drop the junction B down.
Down where? It's not fitting through either of those holes.
Edit - down the top of the seat tube. Sorry, I'm a bit slow today.
@Brommers started
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You only need to get the plugs through the holes.
Use extra long cables and route them all out the top of the seat tube, plug them in, then drop the junction B down.