Never done this particular conversion, but ther eis very few wires to connect engine loom into car loom.
Just the following should get you somewhere permanent live(red, or red/blue), switched live(black), earth(brown), fuel pump output, may be black/red, but will be black/colour, then thin green tacho, brown/white temp gauge.
The ecu loom normally has a plug near the main ecu conenctor where it plugs into the car loom to get these wires hooked up, it will normally be 5 or 6 wire connector, and on sierra at base of windscreen area(inside car) tucked up behind the main loom wher eit is secured to the bulkhead with the plastic clips.
tabetha