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.