70 Dart GT Bulb holders

Plastics get brittle with age. These bulb sockets break. Nothing new.
Best plan is replace all the sockets at once and be done for several years. You can get them new off the HELP line at most parts stores. These aren't by application but by size. You need the 5/8 sockets for your mopar
Same socket under a NAPA part number might be made of a better plastic but they'll cost more too.