Problems with radio

[ma444]

The 'aftermarket' Sony stereo in my son's Polo has stopped working. When you turn on the ignition, the unit lights up, and there is no sound coming from the system.

when you plug in headphones the system works fine so it seems the unit itself if fine.

I have tried another head unit and this has the same issue.

I put in the the VW radio which I took from the car a few years ago. The drivers side (right hand side) speaker works and sound comes out of it - but there is no sound from the left hand speaker on the vehicle.

Does anyone have any ideas as to what is the reason - any advice as to what to look for would be appreciated.