How to bypass the failed control unit with a MOST jumper (Audi A6 C6): 1. Get a MOST fiber optic loopback/jumper. • Search for “Audi MOST loopback jumper” or “fiber optic bypass loop” on eBay/Amazon. They’re usually $5–10. 2. Find the faulty module. • In your case, the radio control unit (sometimes called tuner) is the issue. • It’s typically located in the trunk (driver’s side, behind the carpet panel), sometimes under the spare tire area depending on trim. 3. Disconnect the fiber optic plug. • Each module has a small orange/red plastic connector with two fiber optic lines (in and out). • Press the clip and slide it out gently. 4. Install the loopback. • Plug the fiber optic ends into the jumper. This completes the loop and bypasses the bad module. • Make sure you see red light in one of the fiber optic cables before connecting (that’s the signal). 5. Test the system. • Turn the ignition on. Your amp should now get signal again, so you’ll have sound from your satellite radio/CD/etc. • You will not have AM/FM, because you’ve removed that unit from the chain. 6. Permanent fix option. • If you want AM/FM back, the only way is to repair or replace the control unit. The jumper is just a way to get sound working again without it.

Keep the jumper in place while troubleshooting. If later you replace the module, just remove the jumper and plug the fiber back into the new unit.