Even when disconnected, car batteries will self-discharge at a rate of 5-15% per month, so after an year, the battery will likely be drained completely.
@LemmyKnowsBest, what you could do is buy a solar trickle charger for your car battery, it’ll help maintain the charge level. They’re fairly cheap and good option, as long as your car (or panel) is exposed to sunlight.
Another issue is that your tyres will deflate. Typically tyres deflate around 1-3 PSI per month, or maybe more during summer or in hot climates. So by the end of the year, your tyres could be deflated to 3/4th their capacity. To solve this, you can get a portable air compressor, which can be powered by your car battery (which is hopefully still charged!).