I have a 99 VW New Beetle. My 4 year old son sits behind the drivers seat in a Century car seat. Because he is now over 40 lbs., I have removed the harness from the carseat and the carseat is used more as a booster seat (the carseat is not attached to the car and - my son and the carseat are secured by the car's seatbelt).

Because the NB has a type of seatbelt that has the ability to "lock" (when
fully extended and then retracked in, the belt locks at intervals, thus not
allowing any slack or forward movement) I was wondering if this "lock"
feature is safer for my son in his car seat? Or should I just use the
seatbelt as it is "normally" used with the booster seat? I would love to
know your thoughts on this.

As a separate question, what about the use of a tether strap that anchors the car seat to the back of the car's backseat? The recommendation seems to be to not use it when the harness is taken off, but this seems to be backwards to me. Isn't that when you would want the seat to be secured in one more way?

Thanks for any info you can provide.