Plastics seem to be the same, Volvo or Mercedes, having owned both. Best cure is probably to go to the hobby store and buy some fiberglass veil, super lightweight glass cloth used to skin foam wings and such on RC planes. That, with some 15 minute epoxy on the inside would probably stiffen the old plastic parts significantly. Just adding epoxy or polyester resin will not do much, if anything. The resin has virtually no strength without the cloth.