this is a pm i just wrote for someone so i thought i'd post it to help anyone else out.
hi mate. the biggest thing is the depth of the intercooler. my one is 65mm deep and i would say that it is the maximum that would fit behind the bumper.
first thing to do is remove the plastic undertray, the inner wheel arches and the bumper. then there is a plastic cowel that sits around the radiators that will need removing and this wont be able to go back on.
i had to move the power steering cooler down and strapped it to the lower rad support bar.
the reccess in the bumper where the fog lights sit had to be slightly cut away to allow the power steering cooler pipes to clear. only need about an inch square cut out and this dosn't affect the strength and you cant see it when the fog light is back in.
the intercooler is mounted under the bumper crush bar, i just drilled 2 8mm holes through from the top and out through the bottom. i used a couple of M8 x 100mm bolts to hold it in place. may need cutting to exact length, depends on thread length on intercooler. also there is a bolt thats attatched to the underside of the crush bar that the powersteering cooler bolted to, left mine in place and just made sure that the top of the intercooler wasn't rubbing on it, if so just space it down a bit with some washers.
the bottom fixings of my intercooler sit almost the same height as the radiator support bar, i just cut some 20mm x 3mm flat bar that i had laying around to 100mm long. i bolted these to the bottom of the rad support bar with 1, 6mm bolt in each. i then pulled the intercooler as far forwards as it would go and drilled these two bars to bolt the bottom of the intercooler to.
as for piping i had to remove all of the standard airfilter and put a 90 deg piece of silicon hose on the intercooler outlets and sat them up at about 45 deg past the std radiator. on the drivers side there is an earth cable mounted to the chassis i had to undo and turn upwards. this allowed a small area for the silicon hose on this side. thats as far as i have got at the mo. the piping on the drivers side where the earth is will need to be squashed a little to make an oval shape. i.m planning on making a template for the piping from plastic water pipes then getting them made and bent into one continuous piece of aluminium pipe that goes from the turbo and the inlet manifold to the silicone hoses on the intercooler.
i have used the car since fitting the cooler and there seems to be no problems with airflow to the radiator,
if you need anymore info just ask and i'll try my best to answer