Go with a 12 inch cold cathode tube. Just like neon, but less breakable. They are used in computer modding. You can pick up a dual 12 inch kit for around 10 bucks.

I like to put one under the dash and one under the seat, then wire them to come one with the interior lights.

http://www.xoxide.com/coldcathodes.html
http://www.frozencpu.com/cgi-bin/frozencpu/lit-32.html