Harbor Freight sells a $15 cheapie solar panel (with diode) that plugs straight in to a car's cigarette lighter that essentially 'solves' most parasitic drain problems, if you park outdoors, at least.

https://www.harborfreight.com/15-watt-solar-battery-maintain...

(Not all cars have ports that are live with the key off, but the panel has other ways to wire it)