I hear that swapping the radio in a 3 will activate the function, as the 'mod' doesn't work for 2008s. A member here put a '06 radio in his '08 and now his trip computer works.
I highly doubt it, but I'm going to try a 2006 Mazda 3 CD/MP3 changer in my CX-7 someday.
If it works at all (trip or even CD/radio functions) I'll post the results.