You can reorder the controllers in Steam Input after launching the game. I have had to do this a few times when the deck controller got put first when I was playing alone and wanted to sit on my couch.

What I have seen previously is that the amount of energy you get from the solar cells that you could fit on the top of the car is really small compared to what it takes to charge the battery.

Since there is minimal benefit, and it’s costly to include them and wire them to the battery, it hasn’t been viewed as worthwhile.