I'm always impressed by this forums ability to be helpful and refer to previous posts!
As for the OP, I would agree with the person who first replied to this thread, the only way I can think of would be to wire it from the dome lights.
In my project, as you can see in the link Atraudes posted, I've wired it to a switch on the dash (which is exactly what you're not interested in).
But the footwell lighting came out quite nicely, Both the driver's side and passenger side have a nice flat area that is perfect for sticking an LED strip on so that it directed downwards at a nice angle for lighting without anyone seeing the source (unless you lean down and look ofcourse).
As for the backseat, it was not as nicely mountable. There are 2 hinges for each of the folding seats, I essentially attached the lights to this and surprisingly they have been holding quite well, and still allows full functionality of the folding seat.
I used 4 LED strips with 15 LEDs each, something like this;
http://www.ebay.ca/itm/4-X-15-SMD-3528- ... Wm&vxp=mtr