Modifying an automotive 12V power outlet to disable power when car is off?

The solution ends up being very, very dependent on the exact model of your car and the physical distribution of the outlets within the vehicle.

You'll need a service manual for your car that has a complete wiring diagram. Check first before you buy one, as many only have block diagrams or step-by-step electrical tests instead of full wiring diagram.

Next, identify the 12V outlets on the wiring diagram. Outlets that are on all the time are likely connected to B+ (straight to the battery) via a 15A fuse. Outlets that are on only with the key will be connected to a relay that will be on with the run/ignition (IGN) or accessory (ACC) key positions. It, too, will have a 15A fuse upstream from the outlets.

EDIT: Even if your car doesn't have switched outlets, chances are that it has something that is switched on only with the key is in Accessory or Run positions. This is where things become very model-specific. If this is a classic car where the key switch directly controls power to the + side of the ignition coil, connect your new relay coil to that terminal. If it's a newer car, look first for an IGN or ACC relay. In the unlikely event you can't find one, find something that is switched on and off with the key, and tap the power to that device to operate your relay. Be sure to have transient protection (flyback diode or snubber) on your relay coil if you do this.

I'd recommend adding a separate relay and fuse in parallel with your existing IGN/ACC relay, then running dedicated 14-12AWG wire to each DC outlet. You can then install an SPDT toggle to switch between the existing (always on) and new (on with key) 12V supplies, with the center switch terminal connected to the DC outlet. Make sure that you use a switch that is rated for 15ADC or better.

I've done this to my last 3 cars. All of them are Ford Escape Hybrids (2008, 2010 and 2012). The for the first car I was in Circuit City in late 2007 to get Bluetooth installed (the 2008 model didn't have SYNC) and I asked the installer to switch the outlet. He asked his boss if it was possible and said sure, for an extra $50.

When I got home I took a look at what the guy did and it was exactly as I expected. He simply took one of the leads to the outlet on the dash board and cut it. Instead of the original source of the wire, he replaced it with a wire that ran to the fuse panel. He stuck the end of that wire into one of the contacts for a fuse that's switched by the engine. He wrapped it around the metal pin of the fuse so that it stayed in. I did the same for my next two cars. It's been very useful.

I previously had foreign cars that had switched outlets. I was very frustrated by American cars where the outlet is always on. Lots of iPhone accessories work better when the outlet is switched because it pauses the music when you turn off the car. Doing this has made driving an American car much more tolerable.