I don't think they have any 3d model access to the other cars in btcc. Maybe the Chevrolet Cruze WTCC that's out for Assetto can be used. As I think it's not that different from the btcc version. Just put on a new skin and physics.
But for the Mercedes a modeler is probably needed who takes on the project. Would be cool do to have the audi s3, proton, vw in it to.