If playback from your selected music source is starting from the beginning each and every time you start your car, it's likely due to a lack of constant power supply from the car to your audio unit. Without constant power supply, the Resume function cannot work properly and other settings might need to be reconfigured every time the car's engine is started.
In order to restore the Resume function, make sure that the yellow cable from the connector entering the unit is connected to the constant, 12 Volt power supply source of the car.
NOTE: The constant power supply and switched power supply wire colour coding may vary depending on the car manufacturer and model. In case of doubt, refer to the car's user manual or alternatively, contact the car's manufacturer or a in-car entertainment installation specialist.