Hi, you will have to replace the complete rear screen I dont think that you could repair it correctly using metal paint.
Poor radio reception is a common problem with the e46 so if its still under bmw warranty have them change it, but dont say that it was damaged, or you could alway make a claim on your car insurance for a new rear screen by saying its broken (provided your policy covers windscreen damage)
Or just have the company that done the damage pay for a new bmw screen from your bmw dealer.