On a diesel, a K&N cone in a good location, with the piping between that and the turbo as short and straight as possible. On the 156, I would have it in the space behind the bumper, in front of the N/S/F wheel. I think you have a power steering cooler there though, but its still probably the best location.
I have had the CDA mounted horizontally on mine, just under the relays that sit in front of the battery. Worked quite well, but that was before I got hold of a GSR intake pipe, which meant I had to go vertical with the CDA again.
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