According to the query on Baidu Youdriving, the specific steps to open a music membership for Mercedes-Benz C260L are as follows:

1. Connect to the Internet: First make sure the vehicle is connected to the Internet, either through a mobile phone hotspot or Home wireless network.

2. Launch the Mercedes-Me app: Use the QR code in the user manual that comes with the car or follow the steps below to open the Mercedes-Me app.

3. Log in to your account: Click "My" in the app, and enter the account and password created when purchasing the car on the login interface, or use the mobile phone's shortcut login method to log in.

4. Find "Audio Service": On the "My" page, scroll down to find "Audio Service" and click to enter.

5. Click "Music Membership": On the "Audio Service" page, select "Music Membership" and click to enter.

6. Select a package: On the "Music Membership" page, select the required package and click "Continue".

7. Confirm purchase: On the pop-up purchase confirmation page, click "Confirm Purchase".

8. Complete the payment: Complete the payment according to the prompts to activate the music membership service.