Adrian,I have fitted quite a few "powerboxes" including X5 3.0D but I am not familiar with the Tunit box!!Do you have some pics of the wiring?
FYI the majority of pboxes connect to your FUEL PRESSURE SENSOR on your rail.This is a sensor with 3-wires.(5 VOLTS,Earth and variable voltage).You need a Multimetre to find the wires voltage.The wires from your Tunit box should connect here but you need to know what wires!! Cant you get a wiring diagram from a Tunit dealer?