Graco Nautilus works!
Thanks to all for the responses and suggestions! I had almost given up and to my own surprise I have found the perfect car seat. I guess I won't have to trade the child, eh, car.
After trying 10+ seats, the Graco Nautilus appears to be made for the back seat of the 3 series convertible. The combination of the shorter back rest (with the headrest of the child car seat coming forward just two inches) and the angle between the base of seat and the back rest are a perfect fit. Most seats are too tall and too reclined and therefore you cannot get the seat under the headrest and into the bucket seat of the convertible.
I have included a pictures of the installation. Couple of hints:
- When installing on the passenger side, it easier to clip in the UAS on the door side first, with the seat just moved to the middle
- The UAS loop on the other side can be accessed easily if the ski pass-through part is removed.
- When tightening the UAS base belt put your own full weight on the car seat and keep working the strap. Even when it seems you are making little progress, the fit will get tighter and tighter.
- I was concerned the top tether would be difficult to tighten, but it was not an issue to loop the strap under the head restraint and tighten it that way.
- The car seat has a reclined mode, you don't want to use this for the BMW.
- Adjust the straps for the child before mounting the car seat, it can only be done when the car seat is out.
- I think it is impossible to install the seat with the top closed. Top open and windows down make life a lot easier.
Thanks to all for the suggestions!