I found a way to do this very easily without altering or cutting the car's stock wiring. This mod. will supply power to all 12v outlets with or without a key in the ignition. Having this feature is handy for a number of reasons. Say, you may want to charge a cell phone at a camp site. I use portable DVD players for my kids and with this mod. I don't have to reset the DVD player every time I turn off the ignition. Someone on this site has mentioned they wanted to use an electric cooler on long road trips. The list goes on and on.
The instruction provided here is specifically for a 2008 Outback. If you have a different model car or model year you will have to identify which fuse(s) powers the 12v outlets. You should read the owner's manual for your car and study the fuse box chart before you begin this mod.
Items needed are:
1) 12" of 12 or 14 gauge electrical wire.
2) electrical wire crimper
3) one crimp style electrical connector
4) a multimeter or a DC electrical probe
5) a soldering kit (soldering iron, flux, solder)
6) electrical heat shrink wrap (optional)
7) one mini fuse holder 'add-a-circuit' by Littelfuse (see attached link)
I found the same part at Autozone for $7.50. There are two kinds, one for large fuses and another one for small fuses. Buy the one that's appropriate for your car's fuse box.
1) identify the fuses that power your 12v outlets. On my car they are #13 and #20. You also need to identify one fuse that is already constantly 'hot' to use as the power feed for your 12v outlets. On my car I chose the one for the brake lights (#8).
2) take a spare fuse (included in the 'add-a-circuit' package) and cut the fuse in half with a heavy-duty scissor or wire cutter. Remove the plastic portion and try not to damage the fuse's metal pins. The purpose of this step is to harvest the metal pins that you'll need later. Trim and clean up the pins with a knife such that the pins are free of burrs.
3) solder a 6" long 10 or 12 gauge electrical wire to the end of the pin. Repeat step for each fuse you want to make it constant 'hot'. On my car I have a total of three 12v outlets controlled by two fuses. I made two 6" sections. Use heat shrink wrap to cover the exposed area of the pin.
4) using an electrical crimp style connector, connect the wire end of the 'add-a-circuit' to the 6" wire section(s).
5) connect the 'pins' on the 6" wires to the fuse terminals that power the 12v outlets. You need to identify which one of the two fuse terminals lead to the 12v (the other one leads to the battery (ignition switched). To do this you'll need a multimeter or a DC electrical probe. The terminal that leads to the 12v outlet is the one that registers 0v with the ignition on. On my car the correct terminals are 1) top one for #13, and 2) bottom one for #20.
6) connect the other end of 'add-a-circuit' into the brake light fuse terminal (#8 on my car). For this step, please follow the instructions provided by 'add-a-circuit'. You'll also need to add two fuses to 'add-a-circuit', one for the brake light (20 amp on my car) and another for the 12v outlets (10 amp).
7) test the operation of the 12v outlets and you're done!