I have a Mk2 rev 1 turbo, took the advice of 5.25", removed door-cards and found a 4" speaker there within a larger circular metal surround, attached by 3 self-tappers.

The original circular holes were about 5 inches across, so I re-drilled and managed to secure the 5.25's, but it looks as if the frames for 5.25's are not standard in Jap imports?

In the whole context this is not a big deal, but it is a real pisser to lose half a day on a couple of speakers!