I found this information in a previous post. Is the Wire Up still wrong when it comes to the volume control? ?This talks about swapping the yellow and the red. ?On the current Wire Up it shows the red is not used.?
As you may have read that the volume control wires are incorrectly shown in the wireup diagram.
Please interchange the yellow and red on the volume control. This may solve the problem.
If it becomes worse then there is some other problem..