Cut the cords that feed that cigarette lighter and wire it into the spot meant for any DC power. itll work when the suns out. All 4 links add up to about $106 before tax.
depends what else you want to use it for. The nice part about charging with solar or from a generator is you will have a controller that wont cook your battery. You could get a 100 watt panel and charge controller for under $200 on amazon. Im sure you could get a smaller one for cheaper that could get you by.
If you want to run some lights, charge some batteries (phones, lights) or even run an inverter, the 100 watt panel is the way to go. But if it is just for a swamp cooler, something small liek this would work.