If you are not able to replace your cable at your point of purchase you can arrange a replacement via Ubisoft Customer Services. Please click to submit your request, if you don't have a Ubisoft account already you will be required to create a Ubisoft account to submit your support ticket, we ask you to use the following options to speed up the time needed to start the replacement process:
Product: Select Rocksmith from the product drop-down list.
Question: Enter a description of the problem including any troubleshooting you have attempted.
Once we have the above information we will contact you and advise you how to proceed with the replacement.
Before proceeding with the steps above, please see this article to help determine if you have a defective cable.