Thank you for asking your question on JustAnswer.
The volume control is one of the many ongoing issues with Fords radios, Unfortunately, the problem is internal and the fix is, you will need to return the unit to the dealer and they send them in for the repair. Often times ,the repair is more costly than a quality replacement after market radio.
We have replaced the radio in the 2004 Ford and still have to volume control issue,
If its a ford radio, it still likely at fault.
You did not help me with a solution, but I will go ahead and give you the $9.