There is a difference between "standard" 12VDC and the 12V of a car. A fully charged battery puts out about 13.2V and the charging system will put out over 14V.
If your LEDs are rated for a 12V automotive system, then DIGItonium is probably right about transients/spikes/droops killing your LEDs. Power filters (same ones you'd use with a stereo) will probably work.
If they are rated for 12VDC, then over-voltage is the problem and DIGItonium's regulator should do the trick. You can also use a dropping resistor in series. You will have to know the current draw of the LEDs and do some calculations.
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