The Peg line(and most other high end seats) has what is termed "LATCH" system for attachment points in most vehicles since 2003. The car seat bases have a special "LATCH" belt right on the base, but the seat bases can also be put in older vehicles with a lap belt as well.
I will tell you this, after just having our first kid in December and trying the 3 series with a car seat, it is very hard and annoying to get a seat in and out of that small back seat. So much so that we just traded in our 325xi on a new 4Runner. I hated to get rid of that car, but there was just no way we were going to deal with that problem every time we took the kid someplace.