When you look in the hole in the dash where the stereo should be on my 6 you see a multipin plug, maybe 20 pins.
That is a "unique" style plug for that model car, go to a car audio shop with the car, let them plug and test one or two of their Mitsubishi leads in (£12 maybe)
Their lead will convert it from whatever car you have to ISO which is a standard format.
You Alpine will most likely have an ISO socket on it so by using the adaptor you just plug the unit in with no cutting, crimping or anything.
A lot of cars now come with ISO plugs as their standard and have moved away from "unique" affairs but I am not sure the Japs have done so yet even on the late 8's ?
JJS