We've found the answer to that issue: Pulse Tech Products' Solar Pulse. It's a handy little solar powered system that not only keeps your standby vehicle battery charged, it also keeps it in shape with a system that keeps the battery plates cleaned up, knocking down the buildup of lead sulfate that can kill off an otherwise good battery.
We're using the 5 watt version; it's but about 9" square and a 1/4" thick, small but powerful for the application. Since we keep our car under a fabric cover, the question of where to mount the solar panel came up. We picked up a small wood pallet and mounted the solar panel to the pallet, aiming the solar panel south for maximum solar availability. The next question that arises: What do you do with the solar panel and the pallet when you want to drive? The system has a quick disconnect we hid behind the car grill. We simply unplug the panel from the car, and off we go.
Installation is straight forward and most any "handy with a wrench and screwdriver" person can do the install. A control module lives permanently under the car hood, and the whole install process took us only about 20 minutes.
Now when we leave the car alone for a few weeks or months, we don't worry about having to disconnect the battery for fear "parasitic" loads will slowly chew through our stored up power. Besides, disconnecting the battery is to shut off the juice to the onboard engine diagnostics computer--and any critical information stored therein could be lost with the disconnect of power over a long term.
Check out Solar Pulse on the pulsetech.net website.