Electronics Gurus: Retained Accessory Power?

After I switch off the ignition, I'd like my radio to stay on until I open the door... and of course, stay off when I close the door again.

It seems like this would be a fairly simple project with one or two relays... maybe a capacitor, now that I think about it.
I just can't figure this out myself. Can someone "connect the dots"?

try looking on www.the12volt.com . i'm sure it's been done over there. if not, they'll be able to help you out.

party1: