RCD510 Media In - Sound Issue - Dodgy MDI Box?

[CraigR]

I believe my RCD510 has had the media in/mdi box retrofitted as there is a box wrapped in foam underneath the head unit with the MDI cord running to the glove box.

Unfortunately, when using the media in function, whether it be iPod or USB, the sound coming out of the left speakers is "incomplete" - I can only hear the backing track of the song, no vocals, no bass.

I have buzzed the wires through from the MDI connector to the head unit connector to check the loom in is tact, and its fine, no indication of a broken wire. I have also just plugged the little patch cables directly into the box, to eliminate the extension cord as the culprit. These patch cables are brand new, to further eliminate those as the problem

I think I've just answered my own question - anyone care to confirm as a new MDI box is circa $250... do I need a new MDI box?

Thanks in advance.

[CraigR]

Quick update. Tried my MDI Box in a friends GTi which worked perfectly, so its not the MDI box. I also tried his MDI box in my car which gave the same problem. So it can only be the loom (which I'm pretty confident is okay as I checked the wires for continuity with a multimeter) or the head unit... Anyone?