One leg goes to the fuel cut-out switch for overboost protection, the other to the dash boost gauge. The leg that goes to the fuel cut-out switch passes through the switch and then onto the pressure transducer. That's the bit that "tells" APC what the pressure is. Be aware, the fuel cut-out switch is often bypassed.
You can see a lot more of what is going on down there by removing the LHS speaker grille. Just unclip the heater flue and pull it to one side. Remove the speaker grille and you should have light and access down to the area.
from the excellent 900 Aero dot com site has some pictures of the relevant bits that you'll find down there.