I have just fitted an external hidden 4 way amplifier to my Amundsen head unit, I now have lots of power for my after market speakers but the fader control is no longer shown on the head unit just balance, is there a way to get this back again?

Thank you in advance for any advice on fixing this

I'm having exactly the same problem using a Pioneer GM D1004 micro amp and a plug and play CT10SK03 harness into my Amundsen MIB2 which others on this forum such as JohnnyType2 have successfully used.  That said, given how much better the system sounds (wow!) I can probably live without the fader...but I'd love to know what's going on.  And yes, the pioneer is set to 4 channel input/output.

I believe you need to add Audison USS 4 between the head unit and the amp. I am installing the stereo on my amp and am facing the same issue. Have ordered the part and will let you know