Strap extension on Maxi-Cosi seats??

Hello, we have the following problem: First of all, we are freshly baked parents (for a week!!), but that’s not the problem. We have a Maxi-Cosi for our son, such a baby shell for infants. Yesterday we wanted to visit our freshly baked great-grandparents briefly, and we really had a problem with the attachment of the shell in the car. At the BMW 5 (type E34) there was neither a chance to attach the Maxi-Cosi at the front nor at the back, because the safety belts simply and easily In the Renault Clio my wife’s back also no chance, in the passenger seat it was just so easy to push back with a seat and so on. It can’t be that we have to buy new cars now, just because the belts are 20-30 cm too short? What do people do with more than one child? Are all 5 BMW drivers childless? That can’t be it, can it?? That’s why my question: Does anyone know companies that make extensions for safety belts, possibly e-mail addresses? I am grateful for every tip. Sincerely, Tanja, Michael and the Dwarf