If you want to change the old stereo to Bluetooth function, you can use Bluetooth headset to modify it. The method is as follows: Take off the Bluetooth headset, and then find a lotus plug audio cable with 3.5 turns, as shown in the figure:
1 and 2 in the picture are plugged into the audio input end of the stereo, and the other end is plugged into the Bluetooth headset, and then the mobile phone controls the Bluetooth headset to play music.