CD player only works when engine off

I recently bought a 1999 306 which had an original cassette/radio which I initially replaced with a new Clarion MP3/CD/Radio unit with replacement connectors to use the steering controls.

After one week and a reasonable length run to the West country I decided to replace the Clarion with an original Peugeot cd/radio ( my choice - I just prefer the look and simplicity of the original unit).

I managed to get a Peugeot cd/radio off ebay and have connected it up using the original wiring.

I wired up the unit last night (with the engine off) entered the code and had radio and cd playing no problems (still with engine off). Being a happy bunny I refitted into the gap, left all alone and went in.

The radio worked fine with the engine off and also when I started the car and still responded to the steering stalk controls.

However, while the cd player also responds to the steering stalk (volume, track selection) it would not play any cds while the engine was running.

When I arrived at work and turned the engine off I tried the cd player again.

Any cd that is put in plays but "freezes" when the engine is started - turn the engine off and it resumes play. If a cd is put in with the engine running it remains unread (even if selecting another track number) and is ejected.

I am at a loss as to why this should be. I do not think that I have damaged any of the wiring when changing units.