Twelve Volt outlet stays on

First check for the fuse for that circuit. On many vehicles, the fuse can be plugged in to two of three terminals in the socket. One is hot with the ignition switch turned on, and one is hot all the time. You move the fuse to one terminal or the other as desired.

I will search for a wiring diagram and see if I can find an easy way to do this.

Well, darn the bad luck. There's no simple way to do this, like moving a fuse. How involved do you want to get? Describing what to do will be a lot more complicated than actually doing it. My diagram only shows two power outlets. Rather than try to rewire them from the back, the easiest will be to remove the 25-amp "Aux Pwr" fuse, then plug in a new fuse holder with a pair of wires. One wire will connect to some other circuit, yet to be determined, that turns off with the ignition switch, and the other wire will plug into the old fuse's socket. The fuse to remove is in the under-hood fuse box by the right front strut tower.

With the Aux Pwr fuse removed, test for voltage on the two terminals in the fuse box. One will have 12 volts all the time. It's the other terminal you want. Crimp a spade terminal onto the end of one of the new fuse holder's wires, then plug that into the fuse socket just identified; (the one that doesn't have the 12 volts on it). I'm trying to find something in that same fuse box that is switched with the ignition switch.

GM used to be famous for having individual terminals in their fuse boxes so owner-installed options could be added. Look for a blank terminal marked, "Ign. +", "Switched B+", or something like that. If you find such a terminal, verify it only has 12 volts when the ignition switch is turned on. That would be where to plug in the new fuse holder's second wire.

If there's no available spare terminal, you can also look for any empty fuse sockets that weren't used on your car. See if any of those have a terminal with 12 volts only when the ignition switch is on. Those will work too. The alternative is to use a special terminal that slides into another fuse socket, then the fuse slides in next to it. I don't like those because the second terminal makes the total too wide and can stretch the socket terminal. That can lead to a poor or intermittent connection later when that add-on terminal is removed. If we have to resort to that, I'll try to find a switched fuse in that same fuse box.