Anyone know of an easy/best way to make the cigarette lighter socket powered when the key is off?

Its a '97 850 Wagon that just sits in the driveway (I try to remember to drive it a few miles every month or so but sometimes I forget) and I'm trying to get a solar panel battery trickle charger working (panel is inside car) but the cigarette lighter is only hot when the key is on. If I have to run extra cabling from the battery to charger then I might as well just buy a much less expensive external AC powered charger. The whole point was to have it self contained in the vehicle as I don't have a garage.

 

You can pull the radio (aftermarket or stock) and "T" into the hot wire for the radios memory and run a wire from that T down to the cigarette lighter. I think it's a spade connector to the cigarette lighter and if you pull the stock wire and tape up the end you can always return it to "switched" later should you or the next owner want. One scotch lock, two feet of wire and a spade connector and you'd be good.
Or if you wanted a project you could mount an auxiliary outlet ($8-$10 with wire leads) inside the glove box and pull the power from the glove box light switch. Just make sure you're pulling for the hot side of the switch.