Not sure if it's the same on your car, but on my S4 I routed the line through the ECU box into the engine bay, making a small cut in the rubber grommet which the engine loom goes through. Obviously you will also need to drill a hole into the moulding which holds the vent. I also drilled a couple of smaller holes and fed a zip tie through the base to hold the wires/vac line in place, otherwise the weight of the gauge would cause the vent to point downwards. If you are after a quick MacGuyver fix, you could discreetly wedge some black electrical tape underneath the vent to hold it in position.
Mine is also pre-facelift. I connected the illumination to the 12V switched terminal under the steering wheel. You may want to add an inline variable resistor to dim the illumination a bit depending on your gauge. I find my Omori gauge much brighter than my other dash instruments.
