It all depends on how long you leave your accordion in your car and also depnds on what the temperature is in the car.
If you leave it in there too long, of course the wax will eventaully seperate from the reeds, but it'll take a long time. If you have a handmade they are screwed in anyway.
But if the wax seperates enough that there are internal leaks, melt it back in place. It's not hard, but you obviously have to be careful not to touch the reeds or leathers.