jray3 wrote:I think that a south-facing parking spot with clear sky views is the best idea thus far. The interior is dark enough that I wouldn't expect any added black material to make much difference.
This is the best option for solar heating. To greatly increasing the effect fabricate some custom insulation panels for the rear and rear side windows (perhaps front side windows too). This could be an external or internal solution depending on your preference. Keep the windshield nice and clean too. Aim the car windshield to the southwest (or for maximum exposure for whatever time of day you plan to leave the parking lot). Best case, you could get into an uncomfortably warm car in cold weather.
Material for the panels could be that reflective bubble sheet plastic material. Silver side to the interior of the vehicle!