1992 r129 rear kids seats help

[mitch05]

Hi folks need some advice.

I purchased an 1992 r129 500sl on saturday and its only a weekend car.
Problem is when I go out on weekend I take my wife and 2 kids. My son is 4 and daughter is 2.
Are there any car seats or boosters that can fit in the rear seats? Spoke to a Britax guy and he knew the car very well and said they as a company dont make seats for the age of my car and with lap belts.
Anyone know anything different? Surely I cant be the only 31yr old with kids that takes them out in an old school sl????