There is possibly a better solution out there using these SPDT relays... check the12volt.com under "retained accessory power"
http://www.the12volt.../page5.asp#drto
This sets up your car so that the stereo gets power even after key is out of ignition -- and it gets cut when you open a door. This uses 3 SPDT relays and a couple of diodes and seems like it is going to work pretty well. I've wired up the relays and am about to install them in the car tomorrow. I tested it using a power supply and it seems like it will work like a charm. For those facing a drop in voltage during start,