I have an '88 420SEL, and use the Century 2000STE (top-rated by Consumer Reports) with great success in the center position.
I use the center lap belt of the 420SEL to secure the seat. My son is about 30 lbs. now, and he actually prefers "Daddy's car" to the minivan (like father, like son?).
Century 2000STE is very inexpensive (bought mine at Target) but rated very highly in safety.
Hope this helps! -- Jim.