rear O2 sensor

[baz4380]

Hi all I'm new to the subbie word and plunged for a 2000 legacy B4 twin turbo it's some mods already done by the previous owner replacement panel air filter, uprated fuel pump and a straight through exhaust system 2" down pipes connected to a 2.5" centre to rear with no cats or back box or rear O2 sensor. I intend to fix a sports cat and a back box as it's a bit loud but and also need them for an mot problem is I can't find any sign of where the O2 was plugged in. I've got hols of a manual and the diagram shows it near the gearbox but nothing there. So question is where does that go back to and where would I get a remap done and a rough price. Any help would be very appreciated