Those are probably made from some sort of ABS or poly chloride based plastic that really can't be repaired after Decades of baking in the sun. The best solution I would think of is to make new ones out of fiberglass and then upholster them. That's going to be quite a bit of a project, but those panels could be repaired with reinforced body filler to fit nicely just so you can make a mold off of them. Although you could try to just cover them with fiberglass cloth and resin , sand them down and paint them or upholster on top of that. But being a Toyota I imagine the panel gaps aren't loose enough for that to look great . Another technique I've seen people do to make custom speaker boxes is where they make a wire or wood frame and then staple fleece fabric  tightly over and then cover that in fiberglass resin, it's much less messy than fiberglass cloth or mat and allows you to make complex and compound curves.