MDI not working properly

Guys my MDI is only working on the left channel, the right is either virtually silent or very, very quiet with distortion. The rest of the output from the headunit (FM/DAB/CD/SD card) seems to be OK. I honestly can't tell if this is a new thing since fitting the Columbus or if the problem was there when I originally installed it as I had never checked the balance/fader settings (which was how I noticed the issue this time).

Any thoughts on what it could be? A quick Google search suggests I'm not the only one to have experienced this or a similar problem.