To be more specific, you need a 530 ohm resistor before each LED. Depending on how many LEDs you are looking to light, you can lower the resistance. A 530 ohm resistor at one watt is perfect for running LEDs in a vehicle.
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If you can't find direct replacements for your lights or you find them too expensive there are tons of lights for automotive/marine LEDs that run on 12V right from the box. Do a search for "12V LED light strips" and you'll find a number of lights like these. They take straight 12v, are often sold by the foot, and if you mount them along the ceiling, under cabinets, etc they can provide a surprising amount of light.
If there aren't replacements, you can buy a bunch of LEDs and wire them yourself. You need to put resistors in the circuit so the LEDs don't burn out. There is a great LED calculator at http://ledcalculator.net that shows you what resistors you need how to wire them up. There's a good instructable called "Rayovac LED Lantern Conversion" that gives useful information. Read the discussions too.
Have you checked to see if there are led replacements for your lights. There are led replacements for lots of lights now.