make leds glow on when headlights turn on

Hey everyone, first time on the 12volt! i have a 2008 civic 2dr ex-l with navi.  I want to add LEDs to the interior and i want them to turn on when i flip on my headlights.  the tricky part is that i want them to turn on in a sort of reverse dimming effect... working up to full intensity instead of down if that helps.  I am pretty good at basic wiring hookups ( amp install, HIDs, speaker installs, etc.) but this seems over my head. i have not looked at LEDs yet so its a pretty open ended situation. 

any help would be great

are you using a premade LED light bar or are you going to build it yourself?This information is provided only as a reference.

All circuits should be verified with a digital multi-meter prior to making any connections.

building the setup myself

how many LEDs are you planning to use and do you know the specs on them?This information is provided only as a reference.

All circuits should be verified with a digital multi-meter prior to making any connections.

http://www.oznium.com/led those are the ones im looking at... i think 20 should do it. i am wanting to softly light the floor of my car and then have a few that sit and glow through the rear speaker cages so depending on how bright these are it may be less. i dont know any specs on them other than what is said on the page but in another section of that site they come prewired with resistors. "Use 390 Ohm resistors for a special project or LED configuration. Use 470 Ohm resistors with Blue, Green, White, U.V., Aqua & Pink LEDs" I am going to use white ones but i am unsure if my project qualifies as "special"

You might not even need resistors... But for starters, get some of the LEDs you intend using. Try one with a resistor... Try others in series - that requires a smaller resistor. Enough in series may not require a resistor but that can be a hot topic (pun intended) - eg four series 3.2V LEDs = 12.8V and should tolerate 14.4V (assuming 3.6V capability) - and if the voltage never exceeds 14.4... taking the dimmer into account.... But you need to experiment with the dimming I think you will find that voltage dimming doesn't work well.... Hence I think a PWM circuit is required (but that's not too difficult if you ca construct simple electronic circuits or program a PIC or CPU).

"3.2V LEDs = 12.8V and should tolerate 14.4V" - this will show my ignorance of such things i'm sure, but say i do this but make a series of 8 for example... 3.2V x 8 = 25.6V. now what? will a 14.4V power source not power this series properly?  I am reading other forums on here to figure this stuff out but i seem to have really elementary questions haha

Further example... what exactly is a PWM circuit? i have seen what they are but not quite sure how they work/what the benefits are to having them for this set up

if simply providing a thread from somewhere will help then thats fine because i definitely want to learn this stuff as well

Will 14.4V power 25.6V? Try 4 x 3.2V LEDS instead. Connect the next 4 LEDs in parallel.