it should fit fine forward facing, not sure which Britax you're talking about, because it'll be tight rear-facing.
We have 8mo twins so I never tried anything in the middle seat, but their infant graco's wouldn't fit (rear facing) unless I was basically eating the steering wheel. So they only go in the truck. Last week we just got the Recaro Signo convertible (still rear facing), I haven't tried it in the IS, let me know if you want I'll try it out. Don't know what year your talking about but you know the 01 IS doesn't have the latch system right? If I was to do it over again I'd have gotten an 02+.