Drivers get reminders to wear a seatbelt, turn off the headlights and close the gas-tank door. Now, they’re also getting alerts to check the back seat for young passengers. As the hot-car death rate remains stubbornly high, new sensors and other reminders are in the works. Nissan will make its alert a standard feature, a step advocates have encouraged. This year, 29 children have died from vehicular heatstroke.