Just a simple spst 12v relay that can handle the radio current which is pretty low (I'd go with a 3 or 10amp unit).
If you want the "retained power" feature it's a bit more work, get a dpdt relay and use the second set of contacts to continue to supply the rely with current, You'll need a couple diodes too. Well, here's the circuit:
______________ no connection
/
+12v bat ______________ /
______________ to radio power
______________ no connection
/
__________________ /
| ______________ to door light switch
|
|
|_____|\|____ 2 - 50 volt (or more) 1 amp (or more) diodes
|/| |
|
acc ______|\|____|_____
power |/| ) || The relay will turn on when the key is
12 v ) || turned to the ACC or ON position and
) || remain on until the car door is opened
coil ) ||
) ||
ground _____________) ||
From: Gary Marten