Does anyone know if there are any other non-GM nav discs that will work with the GMT800 TNRs? I've got a 4.1 disc that came with my TNR, but I'd like to see if I can find one that's newer and/or better than the ones GM Supplies, if it's at all possible. I've read about people complaining about the mapping abilities and map accuracy, but the abilities are only as good as the data on the disc. The radio supplies GPS data, which is applied to the data on the disc. If the data on the disc is inaccurate or of poor quality, then the mapping will be inaccurate.