Here's a quick-tip for you:
IKEA offers some solar powered LED garden lights, called SOLARVET. All things considered, those are really inexpensive (even less expensive in Europe, it seems).
I will use those behind the windscreen, inside the van. This way, I can blind the solar cells, forcing the thing to illuminate. Also, I will insert a switch between the main unit and the LEDs, thereby adding control over the illumination provided by the LEDs.
For larger vehicles, there is a 24 LED version of SOLARVET available.