A friend of mine has a 1966 Chevy truck at his ranch that he only uses during deer season. He keeps the battery up with a solar cell that trickels in power through the cigar lighter.

I don't know if this would work for you or not. His truck sits outside and has for about 30 years, so it works for him.

His truck also looks like it has been outside for 30 years, but his battery is always charged!