On your description I noticed that your solar cells are placed to totally obscure the rear view of the driver. Why not place them on louvers instead? You know, something like what Deloreans had over their back windows in the past? Covering the rear window with a louvered set of horizontal "shelves" will shade the interior, allow the driver to see out the back with no difficulty, and allow a flatter surface and thus a more efficient and denser placement of the solar cells. I am uncertain of the aerodynamic cost of louvered rear hatches, but since the planes are in line with the air stream, I'm betting that the effect will be minimal or perhaps even beneficial!
If solar panels mounted on exterior louvers are problematic, interior louvers would also work and maintain the stock aerodynamics. I'm betting that allowing rearview mirror sight out of the rear window would outweigh a small aerodynamic drag (exterior) or loss of interior volume (interior).