O.K. why does this happen?

Since buying my MY00 and fitting a new head unit I noticed that the stereo ran on for a few seconds after switching off / taking keys out. Didn't really bother me but it did mean every time a new passenger took a trip they said 'you've left your..... oh, it's gone off now?!?' Recently I purchased a radar detector to give me a sporting chance against the Herts mobile cash machines and if this, or mobile phone charger, are plugged into ciggy lighter socket the stereo goes off straight away!

Any ideas?

 

see post in general - turn the wipers on ....