Pics:
My 2001 RS4 B5. BMW car seat, ISOFIX. Very solid and rigid when installed, good head restraint, good height adjustable for top cross belt, Only good as a booster seat, rubbish for smaller kids.

My wife's car 2003 B6 Avant:
Rear seat brackets installed by the Recaro Disti's in Ponsonby/Grey Lynn (cant remember name). Very tidy install. Req'd for the Britax Overstrap.

Picture of our Britax seat for our 3 year old. Has both Harness (for 1yr+, but deinstalled in this picture) and support for std 3 point belt. Has a great feature when used with 3 point belt with a centre crutch belt which hooks over std 3 point belt. Anyone with racing experience would know submarining in a car accident is very common, and this stops this. We're using a Britax retro fit ISOFIX strap to keep the seat in place. Pretty solid.

Picture of our Britax seat for our 1 year old. Still using the infant harness. We're also using the Britax retro fit ISOFIX belt. You'll notice the red clip / clamp for when using the seat belt to hold the seat in place. This extra clip makes a big difference when using the seat belt (non ISOFIX installs) and helps stop it getting slack and loose. Highly recommended for non ISOFIX installs.

Close up of the Britax retro fix ISOFIX belt, that allows their typical seat belt attached seat to be ISOFIX attached. You also need the 3rd top strap thou, same as for the ISOFIX install.

I previously mentioned that car seats comply with one of 3x standards. The AU is the highest, and is tested to more G's. Thus the need for the top strap.
When installing the seats, its recommend to get the belts REALLY tight. Put you knee into it, and apply your entire eight when pulling the straps right. When the kids are in the seat, also make sure the belts are tight, they dont like it, buts its good for them if worst case happens.
This sites has some good tips:
http://www.childrestraints.co.nzIf in doubt, visit the distributor for your seat, i've see some rubbish mechanic advice and installs over the years.