[Overdrive]

To the original poster: I don't know about fitting an entire set, but you should be able to at least get 3 in the way you described (frunk and passenger seat). The trunk is really pretty unable to fit many things conveniently. I believe the car was designed around you being able to put any one (flat) front or rear tire in the frunk after putting your spare on the car to get you home. I imagine a front can fit without the need to remove the spare, but a rear will only fit in the frunk by itself (ask Jäger). I doubt a change was made in its lifespan, but since a 17 barely fits in there I imagine an 18, which the car was never designed around to begin with, might not fit in there unless the wheel + tire combination is the same as that of a 17, and I'm not sure how likely that is.

recycledsixtie just made me think of something though. You may want to disable the spoiler (pull the fuse) if you're tying down the trunk lid in such a way that your tie downs are laying over the spoiler. And if you didn't already know, our trunk lids, front and rear, are awfully delicate and prone to dents and warping, so don't put too much force on them.