2003 Citroen C5 rear lighting fault

I recently changed the loom to the nearside rear lights due to the break/side holder being broken.After fitting a brand new loom the break light refuses to work when the engine is running,but with only the ignition on both the side light and break work independently ,but with the ignition on the break works not at all and the side light goes out as well.My local dealer wont help unless I spend &48 per half hour to just look at it so I am now at my wits end as an MOT is due very shortly.Please help.